Gas decomposition apparatus and method for decomposing gas

ABSTRACT

[Object] To provide a gas decomposition apparatus and a gas decomposition method in which no safety problems occur in spite of the application of a relatively high voltage between an anode and a cathode for the purpose of decomposing odorous gases of many types. 
     [Solution] A catalytic electrode layer  6  that contains a catalyst and is porous; a counter electrode layer  7  that forms a pair with the catalytic electrode; and an electrolyte layer  15  that is sandwiched between the catalytic electrode and the counter electrode and has ion conductivity are included. The catalyst is held by the catalytic electrode in the form of being carried by a carrier containing a conductive material or the catalyst is directly carried by the catalytic electrode. A conductive material in the catalytic electrode, the conductive material being in contact with the catalyst, is not a noncovalent carbon material.

TECHNICAL FIELD

The present invention relates to a gas decomposition apparatus and amethod for decomposing a gas. Specifically, the present inventionrelates to a gas decomposition apparatus and a gas decomposition methodfor decomposing and deodorizing an odorous gas through anelectrochemical reaction.

BACKGROUND ART

To decompose odorous components contained in the air by using electricalenergy, a deodorization apparatus is proposed in which ahydrogen-ion-conductive electrolyte layer is sandwiched between twoelectrodes, one (anode) of the electrodes is provided with a gasintroduction path, and a voltage is applied between the anode andcathode electrodes to decompose odorous gases (Patent Literature 1).According to this deodorization apparatus, a voltage is applied betweenthe two electrodes so that odorous gases such as acetaldehyde aredecomposed and deodorized through an anode reaction. As for thisdeodorization apparatus, an example in which sulfuric acid is used as anelectrolyte and an example in which an ion-conductive resin havinghydrogen-ion (proton) conductivity is used as an electrolyte aredisclosed. Here, the electrode is formed by applying fine catalyticparticles of platinum, ruthenium, iridium, or the like carried by acarbon powder to a porous carbon substrate and firing the resultantporous carbon substrate. Thus, odorous gases such as acetaldehyde,formaldehyde, ethanol, and methanol can be decomposed.

The above-described sulfuric acid is a well known electrolytic solution.The above-described ion-conductive resin is a well known polymeric resingenerally referred to as a “perfluorocarbon (PFC) cation exchangepolymer”. As for PFC polymer membranes in which sulfonic acid groups andcarboxylic acid groups are used as ion exchange groups, for example,there is “Nafion” (registered trademark) manufactured by E.I. du Pont deNemours and Company. Since such PFC polymers lose their ion conductivitywithout moisture, moisture is necessarily required.

Citation List Patent Literature

PTL 1: Japanese Patent No. 2701923

SUMMARY OF INVENTION Technical Problem

In the above-described cases, sulfuric acid or a PFC polymer is used asan electrolyte and a voltage of 0.8 V is applied between the twoelectrodes. By applying such a voltage between the electrodes, in theabove-described gas decomposition apparatus, sulfuric acid or a PFCpolymer serving as an electrolyte is kept in a stable state andacetaldehyde, formaldehyde, ethanol, and methanol that are odorous gasescan be decomposed. As for ethanol to be decomposed with thedeodorization apparatus, the decomposition voltage is 1.3 V. However, asdescribed above, the decomposition of ethanol proceeds under theapplication of a voltage of 0.8 V. The magnitude of voltage that shouldbe applied between the anode and the cathode to decompose a gas variesdepending on the type of the gas to be decomposed and also on the typesof electrodes and electrolytes. This has not been completely clarifiedyet.

Among odorous gases, there are gases of aromatic compounds such asbenzene and toluene that have higher decomposition voltages. Thedecomposition of such aromatic compound odorous gases does not proceedunless a higher voltage is applied between the electrodes. For example,toluene has a decomposition voltage of 2.5 V. However, it is not knownwhat problems are caused by the application of a higher voltage betweenthe electrodes. Fuel cells, which are different from gas decompositionapparatuses in that gas decomposition apparatuses are related toelectrical energy input and fuel cells are related to electrical energyoutput but do share basic chemical reactions with gas decompositionapparatuses, have been actively studied and developed. In contrast, gasdecomposition apparatuses have many unknown problems. In most cases, gasdecomposition apparatuses are used, for example, in places crowded withmany people and indoors where sufficient air circulation is notexpected. Accordingly, safety problems that can be caused by theabove-described application of high voltage between the electrodes mustbe meticulously addressed.

Although ethanol has a higher decomposition voltage than water, ethanolis decomposed with the above-described deodorization apparatus with avoltage of 0.8 V between the two electrodes. Thus, this decompositioncan be performed without affecting a PFC polymer that necessarilyrequires moisture or affecting sulfuric acid, which is an aqueouselectrolyte. However, when it comes to the above-described gases ofaromatic compounds such as benzene and toluene that have higherdecomposition voltages, a higher voltage is applied between theelectrodes. The higher the applied voltage is, the higher thedecomposition rate of odorous components having low decompositionvoltages becomes. However, when a voltage equal to or higher than thedecomposition voltage of water cannot be applied, the decomposition rateis limited.

Although it cannot be said that PFC polymers are classified as aqueouselectrolytes, as described above, PFC polymers lose their ionconductivity without moisture. Thus, when a voltage equal to or higherthan the decomposition voltage of water is applied, some problems may becaused. For example, there may be cases where PFC polymers are used andrapid decomposition of odorous gases of many types including aromaticcompounds by the application of a voltage equal to or higher than thedecomposition voltage of water between the two electrodes is notachieved. For example, when PFC polymers are used as electrolytes, inspite of the necessity of the application of a high voltage for rapiddecomposition of aromatic compounds such as toluene, there may be caseswhere constraints are added, for example, the voltage applied betweenthe anode and the cathode cannot be increased beyond a predeterminedvalue under a long-term use or in dry environments. In addition, whenaqueous electrolytes or electrolytes that necessarily require moistureare used, the application of a voltage equal to or higher than thedecomposition voltage of water results in decomposition of water usingthe supplied electrical energy. Thus, energy efficiency is degraded anda decrease in the decomposition rate is unavoidable. Such problems areclearly not desirable for performing rapid deodorization or achievinghigh energy efficiency.

An object of the present invention is to provide a gas decompositionapparatus with which, under the application of a relatively high voltagebetween the anode and the cathode, odorous gases of many types can berapidly decomposed with high energy efficiency without causing safetyproblems. Here, it is appropriate to make the relatively high voltage tobe about 0.8 to 1 V or higher in view of the relationship between theconcentration of carbon monoxide and voltage described below (refer toFIG. 2). In this case, in view of the experimental technique, thevoltage of about 0.8 to 1 V or higher is the output voltage of a voltagesource. That is, the voltage corresponds to the nominal voltage of abattery.

Solution to Problem

A gas decomposition apparatus according to the present inventionincludes a catalytic electrode that contains a catalyst and is porous; acounter electrode that forms a pair with the catalytic electrode; and anelectrolyte that is sandwiched between the catalytic electrode and thecounter electrode and has ion conductivity. The catalyst is held by thecatalytic electrode in a form of being carried by a carrier containing aconductive material or the catalyst is directly carried by the catalyticelectrode. A conductive material in the catalytic electrode, theconductive material being in contact with the catalyst, is not anoncovalent carbon material.

In the above-described configuration, even when a high voltage withwhich aromatic compound gases are decomposed is applied between thecatalytic electrode and the counter electrode, since noncovalent carbonis not in contact with the catalyst, carbon monoxide is not generated.Accordingly, while aromatic compounds are rapidly decomposed, safety canbe ensured in that no carbon monoxide is generated. In addition, forexample, odorous gases having low decomposition voltages can bedecomposed at a high rate by the application of the high voltage. Thus,odorous gases can be decomposed in a short period of time from theinitiation of the operation of the gas decomposition apparatus. Thenoncovalent conductive carbon materials are carbon materials such ascarbon black, graphite, and acetylene black (corresponding to the porouscarbon substrate and the carbon powder disclosed in Patent Literature 1described above). In such noncovalent carbon materials, carbon atoms arebonded together through noncovalent bonds. As indicated by the databelow, these bonds are broken with use of a voltage source having anoutput voltage of 0.8 to 1 V or higher in the presence of a catalyst tocause an oxidation reaction to proceed. The “conductive material that isnot a noncovalent conductive carbon material” may be any metal materialor a covalent carbon material containing an impurity at a highconcentration (for example, conductive diamond or the like). Theabove-described output voltage of a voltage source is not excessivelyhigh. To date, cases where such a voltage source is used and a carbonmaterial such as carbon black is used as a catalytic electrode have notbeen recognized as issues. This is probably due to the followingreasons.

(1) To date, the generation of carbon monoxide has probably not beennoticed. That is, there has been a possibility of the occurrence ofcarbon monoxide poisoning; however this has not been recognized.(2) Such apparatuses are basically not for general purpose use and asmall number of apparatuses have been in operation. The application of anominal voltage does not mean that a voltage that is exactly the nominalvoltage is applied to an intended point, and the applied voltageconsiderably varies in accordance with individual electrochemicalsystem, internal resistance, and the like. Accordingly, in such a smallnumber of apparatuses being in operation, a relatively high voltage hasactually not been applied to noncovalent carbon materials used forcatalytic electrodes.

The conductive material in the catalytic electrode may be a porous metaland at least a portion of the porous metal may be formed by a platingprocess. As a result, the proportion of pore portions can be increasedand metal plated portions can be made small. Thus, porosity can beselected from among a wide range of values so as to be large.Accordingly, gas to be decomposed can be passed through the catalyticelectrode relatively smoothly (at low pressure loss). At this time, thegas does not flow in the form of a laminar flow stagnating in thesurface layer of the catalytic electrode but flows in the form of aturbulent flow with gas separating from the surface of the catalyticelectrode and new gas being supplied to the surface of the catalyticelectrode. Accordingly, by applying a higher voltage, the decompositioncan be performed more efficiently. That is, by passing gas through aporous metal entirely formed by a plating process, a gas flow with whichthe decomposition efficiency can be enhanced under the application ofhigh voltage can be obtained. As a result, while the generation ofcarbon monoxide is prevented, a higher voltage can be applied todecompose gas components at a higher decomposition rate. In particular,as described below, unlike fuel cells configured to generate electricpower by decomposing gas at a high concentration, such a configurationis advantageous for decomposing gas components at a low concentrationefficiently.

Whether a porous metal has been formed by a plating process or not canbe determined by, for example, observation of sectional texture with anoptical microscope and composition analysis of trace components byvarious solid state spectroscopic techniques. In particular, forexample, it is considerably easy to identify that compacting andsintering of powder in which plastic flow due to mechanical processingis generated have not been performed and that casting in which atemperature gradient is unavoidable has not been performed.

The catalyst may be carried by the porous metal through a resin havingproton permeability. In this case, an anode that is a catalyticelectrode can be prepared by mixing fine catalytic particles or a powdercarrying fine catalytic particles with a binder resin having protonpermeability, applying the thus-mixed binder resin to the porous metal,and drying the resin. As a result, a gas decomposition apparatus havinggood electrical or ion conductive continuity to an electrolyte andhaving a low internal resistance can be produced.

The porous metal may have a porosity of 0.6 or more and 0.98 or less. Inthis case, a flow can be obtained in which gas components to bedecomposed are passed through the catalytic electrode relativelysmoothly and separated from the surface of the catalytic electrode andnew gas components are supplied to the surface of the catalyticelectrode. Accordingly, by applying a higher voltage, a gas flow withwhich a high decomposition rate is achieved can be provided. As aresult, while the generation of carbon monoxide is prevented, thedecomposition efficiency of gas components can be enhanced by applying ahigher voltage. When the porosity is less than 0.6, a smooth flow ishampered and passing of gas requires a large amount of energy. When theporosity is more than 0.98, the proportion of gas having passed withoutbeing subjected to a decomposition reaction increases. Thus, by takingin the gas again and decomposing the gas, the concentration of odorouscomponents in a target space is decreased.

The porous metal may be formed by plating a resin formed by a foamingtreatment for forming a large number of bubbles in the resin and ajoining pore-formation treatment for joining the bubbles to form pores.In this case, a porous metal can be readily and efficiently obtainedwith a resin such as urethane or melamine. A porous metal formed byplating a resin formed by a foaming treatment and a joiningpore-formation treatment can be made to have small pores and smallframes. Accordingly, the above-described flow of separation and supplyof new gas in the surface of the catalytic electrode can be locallygenerated within a micro-area. That is, such turbulent flows having asmall size can be locally generated at a high density. As a result, inthe catalyst disposed at a high density, the proportion of the catalystthat is in operation at a high efficiency can be increased. Such aneffect is considerably advantageous for gas decomposition apparatuses,which are required to efficiently decompose odorous components at a lowconcentration unlike fuel cells.

When urethane is used as the resin, 400≦(x−0.3)y can be satisfied wherethe pore size of a porous metal is defined as x (mm) and the specificsurface of the porous metal is defined as y (m²/m³). In this case, gasflow can be made smooth and a flow with which a high reaction efficiencyis achieved can be provided with certainty.

A voltage source with which a voltage of 0.8 V or higher can be appliedmay be provided. According to experiments performed by the inventors ofthe present invention, when a voltage source having an output voltage of0.8 V and a catalytic electrode mainly constituted by a noncovalentcarbon material are used, the danger of carbon monoxide needs to beaddressed in compliance with strict safety standards. Thus, when avoltage source having an output voltage of 0.8 V or higher is used, thepresent gas decomposition apparatus can exhibit the advantage of beingsecure and safe. For example, this is effective when a battery having anominal voltage of about 0.8 V or higher is used.

The following configuration may be employed: a voltage source with whicha voltage of 0.8 V or higher can be applied between the catalyticelectrode and the counter electrode is provided; and, in a state inwhich a voltage of 0.8 V or higher is applied between the catalyticelectrode and the counter electrode by using the voltage source, carbonmonoxide is not generated. In this case, odorous gases having lowdecomposition voltages can be decomposed at a high decomposition ratewith a voltage of 0.8 V or higher without the possibility of carbonmonoxide. Note that, in the state in which a voltage of 0.8 V or higheris applied between the catalytic electrode and the counter electrode byusing the voltage source, it is not necessary that 0.8 V or higher isactually applied between the catalytic electrode and the counterelectrode. The above-described application of a voltage should beunderstood to represent an operation of applying a voltage between thecatalytic electrode and the counter electrode by using a voltage sourcehaving an output voltage of 0.8 V or higher.

A voltage source with which a voltage of 1.5 V or higher can be appliedmay be provided. In this case, odorous components of more types can bedecomposed without the possibility of the generation of carbon monoxide.In addition, odorous components not having very high decompositionvoltages can be decomposed at a high decomposition rate.

The following configuration may be employed: a voltage source with whicha voltage of 1.5 V or higher can be applied between the catalyticelectrode and the counter electrode is provided; and, in a state inwhich a voltage of 1.5 V or higher is applied between the catalyticelectrode and the counter electrode by using the voltage source, carbonmonoxide is not generated. In this case, the decomposition of aromaticcompounds having high decomposition voltages can be rapidly performedwith a high voltage of 1.5 V or higher. In addition, the decompositionof odorous gases having low decomposition voltages can be performed at ahigh decomposition rate. Furthermore, to decompose odorous gases havingrelatively high decomposition voltages more rapidly with certainty, thevoltage of the voltage source is preferably set at 2.0 V or higher. Notethat, in the state in which a voltage of 1.5 V or higher is appliedbetween the catalytic electrode and the counter electrode by using thevoltage source, it is not necessary that 1.5 V or higher is actuallyapplied between the catalytic electrode and the counter electrode. Theabove-described application of a voltage should be understood torepresent an operation of applying a voltage between the catalyticelectrode and the counter electrode by using a voltage source having anoutput voltage of 1.5 V or higher. Such an understanding is common inthe present field. In gas decomposition apparatuses, the electricalresistance of the electrolyte and the electrical resistances at theinterfaces of the catalytic electrode/electrolyte/counter electrodeconsiderably vary. Even in the same products, these values vary inaccordance with production chance and from lot to lot. It is known that,even when a voltage source has a nominal voltage of 1.5 V or higher, thevoltage actually applied between the catalytic electrode and the counterelectrode by using the voltage source is smaller than the nominalvoltage due to various factors.

The conductive material being in contact with the catalyst may belimited to metal and/or conductive diamond. By not using noncovalentcarbon fiber or the like and by using a porous metal or a porousmaterial in which a conductive-diamond thin film is formed on a poroussubstance, the gas decomposition action is achieved and the possibilityof the generation of carbon monoxide can be eliminated. Even whennoncovalent carbon paper or acetylene black is used, as long as aconductive-diamond thin film is formed on the surface of such anoncovalent material, the generation of carbon monoxide does not occur.Thus, the core material may be a noncovalent carbon material. Theconductive diamond may have any form as long as the conductive diamondhas conductivity due to an impurity contained therein at a highconcentration and bonds between carbon atoms are not noncovalent. A thinfilm of covalent diamond or a carbon material referred to as a diamondlike material that can be substantially seen as being covalent may beemployed, the thin film containing an impurity at a high concentration.In particular, conductive diamond doped with boron is preferred.

The carrier may be a conductive-diamond-coated powder that is a powdercoated with conductive diamond; or a metal powder. In this case, areaction in which carriers are used as the carbon source of carbonmonoxide does not occur. Thus, aromatic compounds can be decomposed witha high voltage without the generation of carbon monoxide. Here, aromaticcompounds are mentioned as gaseous compounds having high decompositionvoltages. However, gaseous compounds having high decomposition voltagesare not restricted to aromatic compounds and the gaseous compounds aregases having decomposition voltages higher than the decompositionvoltage of water and having so high decomposition voltages that thegases are not decomposed unless a voltage at which CO can be generatedat a carbon electrode in the presence of a platinum group catalyst isapplied.

The catalytic electrode may include a conductive-diamond-coated poroussheet that is a porous material coated with conductive diamond; or aporous metal sheet. In this case, a reaction in which the sheet materialof the catalytic electrode is used as the carbon source of carbonmonoxide does not occur.

The catalyst may be a platinum group catalyst. In this case, catalyticaction excellent in terms of an oxidation reaction can be achieved andgas decomposition can be promoted. Although such a catalyst also hascatalytic action for the generation of carbon monoxide, as describedabove, noncovalent carbon materials are not in contact with the catalystand hence the generation of carbon monoxide is suppressed. The platinumgroup catalyst includes one or more of platinum group elements(ruthenium Ru, rhodium Rh, palladium Pd, osmium Os, iridium Ir, andplatinum Pt).

The electrolyte may mainly include a uniaxially or biaxially stretchedporous polytetrafluoroethylene (PTFE) membrane and a perfluorocarbon ionexchange polymer (PFC polymer) that fills gaps of the porous PTFEmembrane and extends from the catalytic electrode to the counterelectrode. In this case, while ion conduction between the electrodes isensured, the PFC polymer can be formed into a thin membrane. As aresult, moisture can be readily maintained, the ion conductivity isenhanced, and the electrical resistance is decreased. However, since thePFC polymer is reinforced by the PTFE membrane, the strength of the PFCpolymer in the presence of moisture can be enhanced. As for pin holes,the fluorocarbon resin fibers of the PTFE membrane interrupt the pinholes or do not completely interrupt but bend or divert the pin holes.Thus, passage of odorous gas molecules having a large average radiusunlike the gas of hydrogen having a small radius can be readily blocked.Accordingly, problems that odorous gases pass by pin holes and henceodorous gases need to be taken in again and decomposed and thedecomposition of the odorous gases takes a long period of time can beovercome. The counter electrode may be formed as an electrode layer onwhich fine metal particles having a catalytic function are carried or anelectrode layer not having such a catalytic function. As for theuniaxially or biaxially stretched porous PTFE membrane, for example, amembrane having a pore size of 5 μM or less and a porosity of 50% ormore and 95% or less is preferably used.

Biaxially stretched porous PTFE has a structure having a higher densityin which fine fibers extend from micro-nodes. The reinforcement of thePFC polymer can be performed with uniaxially stretched porous PTFE.However, since the density of micro-nodes and fibers extending from themicro-nodes is considerably high in biaxially stretched porous PTFEcompared with uniaxially stretched porous PTFE, the reinforcement effectof biaxially stretched porous PTFE is considerably high. As a result, toreadily maintain moisture, to enhance ion conductivity, and to decreaseelectrical resistance, the thin membrane can be formed. That is,resistance to a decrease in the strength and to pin holes in the case ofa membrane that is thin and moist can be considerably enhanced. As aresult, the efficiency can be ensured and the durability can beenhanced.

When the porous PTFE membrane has a porosity of less than 50%, theamount of the PFC polymer that conducts ions is insufficient and theelectrical resistance of the electrolyte is increased. Then, to reduce adecrease in the efficiency, the voltage applied between the electrodesis increased, which is not preferred. When the porosity is more than95%, the reinforcement with the porous PTFE membrane is insufficient.For example, leakage of odorous gases through pin holes increases andthe efficiency of deodorization is degraded.

The electrolyte may have a thickness of 50 μm or less. As describedabove, the reduction in the thickness of the electrolyte constituted bya porous PTFE membrane and a PFC polymer is very preferable in view ofachieving a sufficiently high efficiency. When the thickness of theelectrolyte is more than 50 μm, the electrical resistance of theelectrolyte is increased and the necessary voltage applied needs to behigh, which hampers the efficiency, size reduction, weight reduction,and cost effectiveness of the gas decomposition apparatus. However, whenthe thickness of the electrolyte is less than 100 nm (0.1 μm), pin holeshaving a large diameter and extending through the solid electrolytelayer are readily formed and leakage of odorous gases tends to occur. Inaddition, even when reinforcement with a porous PTFE membrane isperformed, it is difficult to achieve a sufficiently high durability inthe presence of moisture. Accordingly, the lower limit is preferablymade to be about 100 nm, more preferably 50 nm, still more preferably 30nm.

A surface of a fiber of the porous PTFE may be covered with ahydrophilic resin film. PTFE is inherently water repellent and repelswater. Accordingly, the integration between PTFE and a PFC polymerhaving water retentivity is not good, which can cause degradation duringusage. However, as described above, by forming a hydrophilic resin film,a large number of micro-nodes and fibers running through the micro-nodesand the PFC polymer are further integrated, that is, the contactresistance between the PTFE and the PFC polymer is increased. Thus, thereinforcing effect provided by the PTFE is considerably enhanced. Inaddition, a large number of micro-nodes and fibers serve as a waterreservoir and can supply moisture to the PFC polymer at the time of thelack of moisture and can absorb water at the time of excessive water.Accordingly, the PFC polymer layer can smoothly exhibit the ionconduction action. As a result, in the state of a thin membrane, amoisture environment is ensured, the ion conductivity is enhanced, andthe electrical resistance is decreased. As a result, while asufficiently high efficiency is ensured, a sufficiently high strengthcan be ensured.

A method for decomposing a gas according to the present inventionincludes applying a voltage between a catalytic electrode containing acatalyst and a counter electrode to decompose a gas. In this method, amembrane electrode assembly (MEA) structure in which a conductivematerial that is in contact with the catalyst in the catalytic electrodedoes not include noncovalent carbon materials is prepared; and the gasis decomposed by applying the voltage between the catalytic electrodeand the counter electrode without generation of carbon monoxide.

According to this method, odorous gases having high decompositionvoltages can be rapidly decomposed and safety can be ensured in that nocarbon monoxide is generated. To more rapidly decompose such gasesincluding odorous gases having relatively high decomposition voltageswith certainty, a higher output voltage is preferably applied.

The gas decomposed may include an aromatic compound gas. In this case,aromatic compound gases such as toluene and benzene can be rapidlydecomposed without the generation of carbon monoxide.

Another gas decomposition apparatus according to the present inventionincludes a catalytic electrode on an oxidation side, the catalyticelectrode containing fine catalytic particles; a counter electrode thatforms a pair with the catalytic electrode; and an electrolyte sandwichedbetween the catalytic electrode and the counter electrode, wherein theelectrolyte is a nonaqueous electrolyte.

In the above-described configuration, since the membrane electrodeassembly (MEA) is formed of, as an electrolyte, a nonaqueous electrolytethat is stable under a voltage higher than the decomposition voltage ofwater, the voltage applied between the anode and the cathode can beincreased and odorous gases of many types can be efficiently decomposedwith certainty. Note that, in this case, the output voltage of a voltagesource provided for the present gas decomposition apparatus is notnecessarily higher than the decomposition voltage of water. This isbecause the operation can be safely performed with a voltage sourcehaving an output voltage lower than the decomposition voltage of waterand the nonaqueous electrolyte. In the present gas decompositionapparatus, by increasing the applied voltage irrespective of thedecomposition voltage of water, the gas decomposition rate of the gasdecomposition apparatus can be increased. The counter electrode layermay be formed as an electrode layer on which fine metal particles havinga catalytic function are carried or an electrode layer not having suchfine catalytic particles.

The electrolyte may include (1) an ionic liquid functioning at roomtemperature or (2) CsHSO₄, a molten salt, or a solid oxide electrolytefunctioning under heating. In this case, the scope of selection of theelectrolyte can be expanded in accordance with, for example, theoperation environment, required performance, or required costeffectiveness of the gas decomposition apparatus. For example, sinceCsHSO₄ can function at a low temperature of about 100° C., CsHSO₄ issuitable for applications in which cost effectiveness and highdecomposition capability are required. Ionic liquids are suitable forapplications in which small size, low power, and the like are of higherpriority than cost effectiveness. Solid oxide electrolytes, which needto be heated to a high temperature of 300° C. or higher, are suitablefor applications in which high decomposition capability, durability,being field-proven, cost effectiveness, and the like are of highpriority.

The electrolyte may be a solid membrane containing an ionic liquid. Inthis case, the structure of the MEA can be simplified. In addition,considerations for leakage of liquid and the like are no longernecessary and safety and reliability can be enhanced. As for such asolid membrane containing an ionic liquid, there is, for example, apolymeric membrane containing an ionic liquid formed by dissolving aresin material in the ionic liquid and polymerizing the resin material.

A voltage source with which a voltage of 1.5 V or higher can be appliedmay be provided. In this case, whether the decomposition voltages arehigh or low, odorous gases of many types can be rapidly made harmless.

A voltage source with which a voltage of 1.5 V or higher can be appliedbetween the catalytic electrode and the counter electrode may beprovided. In this case, odorous gases such as aromatic compounds can berapidly decomposed. In particular, when rapid decomposition of odorousgases is required, a voltage source of 2.0 V or higher is desirablyprovided. Note that, in the state in which a voltage of 1.5 V or higheris applied between the catalytic electrode and the counter electrode byusing the voltage source, it is not necessary that 1.5 V or higher isactually applied between the catalytic electrode and the counterelectrode. The above-described application of a voltage should beunderstood to represent an operation of applying a voltage between thecatalytic electrode and the counter electrode by using a voltage sourcehaving an output voltage of 1.5 V or higher. Such an understanding iscommon in the present field. In gas decomposition apparatuses, theelectrical resistance of the electrolyte and the electrical resistancesat the interfaces of the catalytic electrode/electrolyte/counterelectrode considerably vary. Even in the same products, these valuesvary in accordance with production chance and from lot to lot. It isknown that, even when a voltage source has a nominal voltage of 1.5 V orhigher, the voltage actually applied between the catalytic electrode andthe counter electrode by using the voltage source is often smaller thanthe nominal voltage due to various factors.

The following configuration may be employed: a conductive material inthe catalytic electrode, the conductive material being in contact withthe fine catalytic particles is not a noncovalent carbon material. Inthis case, when an external voltage is applied in the state in which thetemperature is increased, the generation of gases such as carbonmonoxide due to the decomposition of noncovalent carbon can beprevented.

A conductive material that is one of main constituent materials of thecatalytic electrode and is in contact with the fine catalytic particlesso that electrons generated by a gas decomposition reaction areconducted through the conductive material may be limited to metal and/orconductive diamond. In this case, odorous component gases can beefficiently decomposed with an existing material without the possibilityof the generation of carbon monoxide.

Still another gas decomposition apparatus according to the presentinvention includes a decomposition-side electrode being a porouselectrode into which gas containing a component to be decomposed isintroduced; a counter electrode that forms a pair with thedecomposition-side electrode and is porous; and an electrolytesandwiched between the decomposition-side electrode and the counterelectrode, wherein a portion of the decomposition-side electrode, theportion being in contact with the electrolyte, is composed of a materialthat is inert to water.

In the above-described configuration, even when a voltage equal to orhigher than the decomposition voltage of water (1.23 V) is appliedbetween the decomposition-side electrode and the counter electrode,water is not decomposed within the tolerance range (within the range ofthe window). Accordingly, when gases to be decomposed having highdecomposition voltages are decomposed, by applying a voltage equal to orhigher than the decomposition voltage of water (1.23 V), the gases canbe decomposed with a high energy efficiency. In addition, gases havinglow decomposition voltages can be rapidly decomposed with a high energyefficiency at a high decomposition rate according to the magnitude ofthe applied voltage.

Note that, in this case, the output voltage of a voltage source providedfor the present gas decomposition apparatus is not necessarily higherthan the decomposition voltage of water. This is because the operationcan be safely performed with a voltage source having an output voltagelower than the decomposition voltage of water and the material that isinert to water. In such a gas decomposition apparatus, by increasing theapplied voltage irrespective of the decomposition voltage of water, thegas decomposition rate of the gas decomposition apparatus can beincreased.

Such a material that is inert to water does not have high catalyticactivity compared with platinum group catalysts. However, suppliedelectrical energy is not used for the decomposition of water and hencethe energy efficiency can be enhanced and a practically feasibledecomposition rate can be achieved. When an aqueous electrolyte or aperfluorocarbon electrolyte is used as the electrolyte, a situation inwhich the lack of water is caused by the electrolysis of water andtarget gases cannot be decomposed can be avoided. Thus, the lastingstability of the decomposition operation can be achieved. It is notnecessary that the entirety of the decomposition-side electrode iscomposed of a material that is inert to water. It will suffice that aportion (such as a surface layer) of the decomposition-side electrodebeing in contact with the electrolyte is composed of a material that isinert to water.

A voltage source with which a voltage of 1.23 V or higher can be appliedmay be provided. In this case, gases having low decomposition voltagescan be rapidly made harmless and aromatic compounds and the like havinghigh decomposition voltages can be decomposed.

The following configuration may be employed: a voltage source with whicha voltage of 1.23 V or higher can be applied is provided; theelectrolyte is an electrolyte containing water; and, in a state in whicha voltage of 1.23 V or higher is applied between the decomposition-sideelectrode and the counter electrode by using the voltage source, thewater in the electrolyte is not decomposed. In this case, aromaticcompounds and the like having high decomposition voltages can bedecomposed with a high voltage, high energy efficiency, and lastingstability.

Note that, in the state in which a voltage of 1.23 V or higher isapplied between the catalytic electrode and the counter electrode byusing the voltage source, it is not necessary that 1.23 V or higher isactually applied between the catalytic electrode and the counterelectrode. The above-described application of a voltage should beunderstood to represent an operation of applying a voltage between thecatalytic electrode and the counter electrode by using a voltage sourcehaving an output voltage of 1.23 V or higher. Such an understanding iscommon in the present field. In gas decomposition apparatuses, theelectrical resistance of the electrolyte and the electrical resistancesat the interfaces of the catalytic electrode/electrolyte/counterelectrode considerably vary. Even in the same products, these valuesvary in accordance with production chance and from lot to lot. Even whena voltage source has a nominal voltage of 1.23 V or higher, the voltageactually applied between the catalytic electrode and the counterelectrode by using the voltage source is often smaller than the nominalvoltage due to various factors.

The decomposition-side electrode may be covered with conductive diamondor may contain any one of a conductive oxide, a conductive nitride, anda conductive sulfide. In this case, a decomposition-side electrode thatis inert to water can be obtained.

The decomposition-side electrode may contain Ti₄O₇ or PbO₂. In thiscase, a decomposition-side electrode that is inert to water can beobtained by using such an available specific material, as well as by thetechnique of covering the decomposition-side electrode with conductivediamond.

The decomposition-side electrode may include a porous sheet covered withconductive diamond on which Ti₄O₇ powder and/or PbO₂ powder is held. Inthis case, the decomposition of odorous components can be promoted.

Another method for decomposing a gas according to the present inventionincludes applying a voltage between a decomposition-side electrode and acounter electrode that sandwich an electrolyte therebetween, andintroducing gas containing a component to be decomposed into thedecomposition-side electrode to decompose the component. In this method,the decomposition-side electrode contains a material that is inert towater and the electrolyte contains an electrolyte containing water; anda voltage of 1.23 V or higher is applied between the decomposition-sideelectrode and the counter electrode so that the component is decomposedwithout decomposing the water in the electrolyte.

According to the above-described method, since target gases aredecomposed with a voltage of 1.23 V or higher, gases having lowdecomposition voltages can be decomposed at a high decomposition rateand gases having high decomposition voltages can be decomposed at apractical decomposition rate. In both cases, the decomposition of waterdoes not occur and hence energy efficiency can be enhanced.

ADVANTAGEOUS EFFECTS OF INVENTION

According to a gas decomposition apparatus and a gas decompositionmethod according to the present invention, under the application of arelatively high voltage between the anode and the cathode, odorous gasesof many types can be rapidly decomposed with high energy efficiencywhile safety can be ensured in that no carbon monoxide is generated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of a gas decomposition apparatus according toa first embodiment of the present invention.

FIG. 2 is a graph illustrating the influence of applied voltage andtemperature on the generation of carbon monoxide when an existing gasdecomposition apparatus is used.

FIG. 3 is a graph illustrating the influence of applied voltage on thedecomposition rate of acetaldehyde decomposed with the same gasdecomposition apparatus as in the experiments in conjunction with FIG.2.

FIG. 4 is a sectional view illustrating a region near the interfacebetween the catalytic electrode and the electrolyte of a gasdecomposition apparatus according to the first embodiment of the presentinvention.

FIG. 5 is a sectional view illustrating a region near the interfacebetween the catalytic electrode and the electrolyte of a gasdecomposition apparatus according to a modification of the gasdecomposition apparatus illustrated in FIG. 4.

FIG. 6 illustrates an electrolyte-side portion of the catalyticelectrode of a gas decomposition apparatus according to a secondembodiment of the present invention.

FIG. 7 illustrates the catalytic electrode of a gas decompositionapparatus according to a third embodiment of the present invention.

FIG. 8 illustrates an example in which carriers are used in thecatalytic electrode of a gas decomposition apparatus according to thethird embodiment of the present invention.

FIG. 9 is a sectional view illustrating a gas decomposition apparatusaccording to a fourth embodiment of the present invention.

FIG. 10 is a schematic view of a stacked structure of a gasdecomposition apparatus according to a fifth embodiment of the presentinvention.

FIG. 11 is a sectional view illustrating a specific example of the gasdecomposition apparatus illustrated in FIG. 10.

FIG. 12 is a sectional view illustrating a region near the interfacebetween the catalytic electrode and the electrolyte of the gasdecomposition apparatus illustrated in FIG. 10.

FIG. 13 is a sectional view illustrating a region near the interfacebetween the catalytic electrode and the electrolyte of a gasdecomposition apparatus other than the gas decomposition apparatusillustrated in FIG. 10.

FIG. 14 illustrates conductive bases of the catalytic electrode of a gasdecomposition apparatus according to a sixth embodiment of the presentinvention; (a) illustrates a conductive base of metal fiber or the like;and (b) illustrates carbon fiber, metal fiber, or the like that has beencovered with a conductive-diamond thin film.

FIG. 15 illustrates catalyst-carrying powders in a gas decompositionapparatus according to the sixth embodiment of the present invention;(a) illustrates a metal powder having been made to carry fine catalyticparticles; and (b) illustrates a noncovalent carbon powder, metalpowder, or insulating powder having been subjected to aconductive-diamond coating treatment and then having been made to carryfine catalytic particles.

FIG. 16 is a schematic graph illustrating voltage-current curvesobtained when an electrode of an existing gas decomposition apparatus isimmersed in water and an electrode of a gas decomposition apparatusaccording to the present invention is immersed in water.

FIG. 17 illustrates an electrolyte-side portion of thedecomposition-side electrode of a gas decomposition apparatus accordingto a seventh embodiment of the present invention.

FIG. 18 illustrates an electrolyte-side portion of thedecomposition-side electrode of a gas decomposition apparatus accordingto an eighth embodiment of the present invention.

FIG. 19 illustrates an electrolyte-side portion of thedecomposition-side electrode of a gas decomposition apparatus accordingto a ninth embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS First Embodiment

FIG. 1 illustrates a gas decomposition apparatus 10 according to a firstembodiment of the present invention. In the gas decomposition apparatus10, a catalytic electrode layer 6 and a counter electrode layer 7 aredisposed with a solid electrolyte layer 15 therebetween. Both thecatalytic electrode layer 6 and the counter electrode layer 7 containfine catalytic particles of platinum or the like. Such fine catalyticparticles may be contained in the form of being carried by a conductivepowder (carriers) in the two electrodes or may be, without carriers,made to directly adhere to (be carried by) electrode sheets by platingor the like, the electrode sheets serving as conductive bases formingthe electrodes. A porous gas-diffusion layer 8 through which an odorousgas to be decomposed is introduced and from which the odorous gas havingbeen subjected to a decomposition reaction (anode reaction or oxidationreaction) is discharged is provided for the catalytic electrode layer 6.The porous gas-diffusion layer 8 is preferably composed of a conductivematerial such as CELMET (registered trademark), which is a porous metalmanufactured by Sumitomo Electric Industries, Ltd. A porousgas-diffusion layer 9 composed of CELMET or the like is also providedfor the counter electrode layer 7 for the purpose of introducing air toprovide oxygen for a cathode reaction and discharging water generated bythe cathode reaction (reduction reaction).

Before the structure of a gas decomposition apparatus according to thepresent invention is described, experimental results serving as thereason why the structure is employed will be described. FIG. 2 is agraph illustrating experimental results for an existing gasdecomposition apparatus. This gas decomposition apparatus includedplatinum serving as a catalyst, carbon black serving as catalystcarriers, carbon paper serving as the conductive base or the poroussheet of a catalytic electrode, and Nafion (PFC polymer) serving as anelectrolyte. The test environment was at room temperature and at a roomhumidity of about 30% to 50%. The higher the temperature and thehumidity become, the more carbon monoxide is generated. The test wasperformed with a cell measuring 3.5 cm per side in terms of the area ofelectrodes by introducing a gas at a predetermined concentration into a3 L tetra bag, circulating the gas in the cell at 0.5 L/min with ametering pump, and appropriately terminating the circulation andmeasuring the concentration of gas such as CO at the outlet side of thecell. As described above, both the carbon black serving as the catalystcarriers and the carbon paper serving as the electrode sheet werenoncovalent. FIG. 2 shows that the application of a voltage of 1.5 Vbetween the two electrodes at 80° C. resulted in the generation ofcarbon monoxide after at least 30 minutes had elapsed. In addition, theapplication of a voltage of 2 V at room temperature resulted in thegeneration of carbon monoxide after at least 30 minutes had elapsed.Sources of the generation of carbon monoxide other than the air are theabove-described carbon black and carbon paper, which are noncovalentcarbon materials. Decomposition of gas that is not limited tohydrocarbons occurs at a position where fine catalytic particles 11, thecatalytic electrode layer 6 (catalyst carriers 21 in the case of usingcatalyst carriers), and the electrolyte layer 15 are in contact witheach other. Carbon monoxide is generated when noncovalent carbon blackor noncovalent carbon paper is present in such a contact position.

FIG. 3 is a graph illustrating the influence of the voltage between theanode and the cathode on the decomposition rate of acetaldehydedecomposed with the gas decomposition apparatus described in conjunctionwith FIG. 2. FIG. 3 indicates that, as the voltage between the twoelectrodes increases from 1 V to 1.5 V to 2 V, the concentration ofacetaldehyde decreases in a shorter time. Accordingly, when a gas otherthan an aromatic compound gas is decomposed, the gas having a lowdecomposition voltage compared with aromatic compound gases, thedecomposition rate can be increased by increasing the voltage appliedbetween the two electrodes.

Features of the Present Embodiment

The gas decomposition apparatus 10 according to the present inventionhas a feature that noncovalent carbon materials such as carbon fibersheets and carbon black are not used as conductive materials with whichfine catalytic particles are in contact. In particular, the presentembodiment has a feature that a porous conductive sheet forming thecatalytic electrode layer 6 is formed of a metal and, when carriers aremade to carry a catalyst, the carriers are formed of a metal. Since sucha porous metal conductive sheet is porous, fine catalytic particles canbe directly carried by the sheet and it is not necessary to preparecarriers such as a powder. However, fine catalytic particles may becarried by carriers and the carriers may be held by the porous sheet ofthe electrode. The term “powder” is a name that identifies theclassification of a product. However, other than such a classificationname for products, the term “powder” also refers to particles having aconsiderably larger size than fine catalytic particles. The electrolytemay be any electrolyte, for example, a PFC polymer that functions atroom temperature such as Nafion or any nonaqueous electrolyte such as anionic liquid. As for such nonaqueous electrolytes, other than ionicliquids, the following electrolytes that function under heating may beused: CsHSO₄, phosphoric acid-based proton conductors such as(NH₄)₂K_(1-x)PO₃, molten salts, and solid oxide electrolytes.Alternatively, phosphoric acid may be used as the electrolyte. Thus, thescope of selection of the electrolyte can be expanded in accordancewith, for example, the operation environment, required performance, orrequired cost effectiveness of the gas decomposition apparatus. Forexample, since CsHSO₄ can function at a low temperature of about 100°C., CsHSO₄ is suitable for applications in which cost effectiveness andhigh decomposition capability are required. Ionic liquids are suitablefor applications in which small size, low power, and the like are ofhigher priority than cost effectiveness. Solid oxide electrolytes, whichneed to be heated to a high temperature of 300° C. or higher, aresuitable for applications in which high decomposition capability,durability, being field-proven, cost effectiveness, and the like are ofhigh priority.

A catalytic electrode containing fine catalytic particles may employ thefollowing structures. (1) As illustrated in FIG. 4, fine catalyticparticles 11 are directly carried on the surface of a porous metal sheet6 m described above. The metal sheet 6 m may be any porous metal such asmetal fiber obtained by processing nickel fine wires, niobium finewires, titanium fine wires, or the like into a fibrous form; a porousmetal material such as CELMET; or a metal sinter obtained by sintering ametal powder.

(2) As illustrated in FIG. 5, a structure in which a conductive metalpowder 21 m of nickel, cobalt, silver, molybdenum, or the like on thesurfaces of which the fine catalytic particles 11 are carried isdispersed in a binder resin having proton permeability and provided onthe surface of the porous metal sheet 6 m can be employed.

As for the catalytic electrode layer 6 in (1) above (although “thecounter electrode layer 7 containing a catalyst” is also included, thedescription thereof is omitted for the sake of simplicity), for example,in the state in which the porous metal sheet 6 m is immersed in asolution containing metal ions forming the fine catalytic particles 11,the metal ions are reduced with a reducing agent to precipitate the finecatalytic particles 11 composed of the metal onto the porous metal sheet6 m. As described above, when a porous metal sheet is used as aconductive base, fine catalytic particles also precipitate onto theinner surfaces of many pores. In such precipitation, fine catalyticparticles are carried by the porous metal sheet 6 m.

A catalytic electrode layer in (2) above is formed as follows. Forexample, the metal powder 21 m of nickel, silver, or the like isprepared. In the same manner as described above, the metal powder 21 mis then immersed in a solution containing metal ions forming finecatalytic particles and a catalyst is precipitated in the form of fineparticles onto the surfaces of the metal powder by using a reducingagent. Such a catalyst-carrying metal powder 21 m, fine catalyticparticles 11 is mixed with a solution of a binder resin having ionpermeability to prepare a coating solution. The coating solution is thenapplied to a surface of the porous metal sheet 6 m and dried to form abinder resin membrane in which the above-described carrier metal powder21 m, fine catalytic particles 11 is dispersed. In the catalyticelectrode layer of (2) above, as described above, porous metal fibersuch as nickel fiber, niobium fiber, or titanium fine wires; a porousmetal material such as CELMET; a metal sinter; or the like is used as aconductive base. In addition, the binder resin membrane is stacked so asto be in contact with the electrolyte.

In such a stack, while the contact between the fine catalytic particlesand odorous components is maintained with the porous conductive base,the catalyst-carrying powder is dispersed in a membrane composed of abinder resin having proton permeability, and the membrane is sandwichedbetween the conductive base and the solid electrolyte. Accordingly, forexample, removal of the fine catalytic particles is suppressed and hencethe catalytic action can be maintained for a longer period of time.

As for the fine catalytic particles, a platinum group element such asplatinum, ruthenium, palladium, iridium, or osmium; an iron group metalsuch as iron, cobalt, or nickel; or a noble metal such as vanadium,manganese, silver, or gold is preferably used. In particular, platinumgroup elements are preferred because they exert an excellent catalyticaction on an oxidation reaction. Alternatively, to enhance a specialfunction, fine catalytic particles composed of an alloy of such metalsmay be used. For example, to enhance a catalyst-poison resistanceserving as a catalytic function, an alloy in which the mass ratio ofplatinum to palladium Pt/Pd satisfies about 7/3 to 9/1 may be used.

In the gas decomposition apparatus 10, by applying a voltage of, forexample, 1.5 to 2 V or higher to the two electrodes, aromatic compoundgases can be rapidly decomposed without the possibility of thegeneration of carbon monoxide. In addition, by applying theabove-described high voltage, not only aromatic compounds but alsoodorous gases having low decomposition voltages such as acetaldehyde andethanol can be rapidly decomposed.

Second Embodiment

FIG. 6 illustrates an electrolyte-side portion of a catalytic electrodeof a gas decomposition apparatus according to a second embodiment of thepresent invention.

The present embodiment has a feature that carriers carrying a finecatalytic particles 11 are coated with a thin film of conductive diamond30 and a metal (porous metal sheet 6 m) is used as a porous sheet forthe catalytic electrode. Cores of the carriers 21 may be any powder anda metal, an insulating material, or a noncovalent carbon material suchas carbon black may be used. In the present embodiment, theconfiguration of the gas decomposition apparatus 10 illustrated in FIG.1 is employed and the porous metal 6 m is used as a sheet material ofthe catalytic electrode layer 6. The porous metal sheet 6 m serving as asheet material of the catalytic electrode layer 6 may be any porousmetal sheet such as metal fiber obtained by processing nickel finewires, niobium fine wires, or the like into a fibrous form; a porousmetal material such as CELMET; or a metal sinter obtained by sintering ametal powder. The electrolyte may be a PFC polymer or any nonaqueouselectrolyte such as an ionic liquid. The catalyst is preferably aplatinum group catalyst in view of promoting decomposition reactions ofgases. However, another catalyst may be used.

As illustrated in FIG. 6, as for the catalyst of the catalytic electrodelayer 6, carriers that are formed by coating the powder (cores) 21 withthe thin film(s) of the conductive diamond 30 and that are made to carrythe fine catalytic particles 11 are used. That is, a structure in whichthe above-described catalyst-carrying carriers 21, 30, 11 are dispersedin a binder resin having proton permeability and provided on a surfaceof the porous metal sheet 6 m can be employed. The powder 21 may be ametal powder, an insulating powder, or a conductive carbon powder ofcarbon black, acetylene black, or the like. When the cores are composedof an insulating material, conductivity can be provided by the thinfilm(s) of the conductive diamond 30 covering the powder 21. As for theconductive diamond 30, covalently bonded carbon atoms themselves do nothave conductivity. However, by making such a material to contain animpurity such as boron at a high concentration, the resultant conductivediamond 30 can have conductivity. The conductive diamond 30 can beformed in the form of thin films on the powder 21 that serve as cores bya microwave plasma chemical vapor deposition (CVD) technique or the likewhile the powder 21 is suspended. At this time, doping with a p-typeimpurity such as boron at a high concentration is performed to impartconductivity.

Carriers that carry fine catalytic particles are formed in the followingmanner. For example, (i) a metal powder of nickel, silver, or the like,(ii) a powder in which conductive-diamond surface layers are formed on ametal powder, (iii) a composite carbon powder in whichconductive-diamond thin films are formed on the surfaces of a conductivecarbon powder such as carbon black, or (iv) a powder in whichconductive-diamond thin films are formed on an insulating powder isprepared. Such a powder 21, 30, 11 is then immersed in a solutioncontaining metal ions forming fine catalytic particles and the metal isprecipitated in the form of fine particles onto the surfaces of thepowder 21, 30 by using a reducing agent (refer to FIG. 6). Thecatalyst-carrying powder 21, 30, 11 is mixed with a solution of a binderresin having ion permeability to prepare a coating solution. The coatingsolution is then applied to a surface of the porous metal sheet 6 m anddried to form a binder resin membrane in which the carrier powder isdispersed. In the catalytic electrode layer 6, the porous metal sheet 6m is used as a conductive base. In addition, the binder resin membraneis stacked so as to be in contact with the electrolyte. In such a stack,while the contact between the fine catalytic particles and odorouscomponents is maintained with the porous metal sheet 6 m, thecatalyst-carrying powder 21, 30, 11 is dispersed in the membranecomposed of a binder resin having proton permeability, and the membraneis sandwiched between the porous metal sheet 6 m and the solidelectrolyte. Accordingly, for example, removal of the fine catalyticparticles is suppressed and hence the catalytic action can be maintainedfor a longer period of time.

In the gas decomposition apparatus according to the present embodiment,the powder 21 is covered with the thin film(s) of the conductive diamond30 and the porous metal sheet 6 m is used for the catalytic electrodelayer 6. Thus, the fine catalytic particles 11 are not in contact with anoncovalent carbon material such as carbon black or carbon fiber.Therefore, by applying a high voltage of 1.5 to 2 V or higher betweenthe anode and the cathode, aromatic compounds can be rapidly decomposedwithout the possibility of the generation of carbon monoxide. Inaddition, gases having low decomposition voltages can be decomposed at ahigh decomposition rate.

Third Embodiment

FIG. 7 illustrates the catalytic electrode of a gas decompositionapparatus according to a third embodiment of the present invention. Thepresent embodiment has a feature that the catalytic electrode layer 6 isproduced using a porous sheet 16 covered with a thin film of conductivediamond 30. The core material of the porous sheet 16 of the catalyticelectrode layer 6 may be any material and a metal, an insulatingmaterial, noncovalent carbon fiber, or the like may be used. In thepresent embodiment, the configuration of the gas decomposition apparatus10 illustrated in FIG. 1 is employed. The electrolyte may be a PFCpolymer or any nonaqueous electrolyte such as an ionic liquid. Thecatalyst is preferably a platinum group catalyst in view of promotingdecomposition reactions of gases. However, another catalyst may be used.

When the porous sheet covered with the thin film(s) of the conductivediamond 30 is a fibrous sheet, it is preferred that the thin film(s) ofthe conductive diamond 30 be formed on threads and the threads be thenwoven together. In the case of a porous metal material such as CELMET,it is preferred that the porous metal material be placed in a plasma CVDchamber, irradiated with carbon plasma, and doped with an impurity at ahigh concentration to form the thin film(s) of the conductive diamond 30both on an outer surface and an inner surface of the porous material. Aporous insulating material can be treated in the same manner. A poroussheet prepared in the above-described manner and illustrated in FIG. 7is used to form the catalytic electrode layer 6. At this time, astructure below is employed depending on whether carriers are used ornot.

(1) As illustrated in FIG. 7, the porous sheet 16, 30 included in thecatalytic electrode layer 6 is made to directly carry the fine catalyticparticles 11.(2) Alternatively, FIG. 8 illustrates an example in which the poroussheet 16, 30 is made to hold carriers 21, 30, 11 carrying the finecatalytic particles 11. The catalyst carriers 21, 30, 11 include thecore materials of the carriers 21 covered with the thin film(s) of theconductive diamond 30 and hence are the same as those in the secondembodiment. As for the carriers carrying the fine catalytic particles11, other than the configuration illustrated in FIG. 8, the metal powder21 m may be made to carry the fine catalytic particles 11 as illustratedin FIG. 5 according to the first embodiment and the resultant metalpowder 21 m may be held by the porous sheet 16, 30 illustrated in FIG.7.

In the gas decomposition apparatus according to the present embodiment,the porous sheet 16, 30 covered with the thin film(s) of the conductivediamond 30 is used for the catalytic electrode layer 6 and the poroussheet 16, 30 directly carries the fine catalytic particles 11.Alternatively, when carriers for fine catalytic particles are used, ametal powder (first embodiment) or carriers in which a powder is coveredwith the thin film(s) of the conductive diamond 30 (second embodiment)are used as the carriers. Thus, the fine catalytic particles 11 are notin contact with noncovalent carbon materials such as carbon black andcarbon fiber. As a result, by applying a high voltage of 1.5 to 2 V orhigher between the anode and the cathode, aromatic compounds can berapidly decomposed without the possibility of the generation of carbonmonoxide. In addition, gases having low decomposition voltages can bedecomposed at a high decomposition rate.

Fourth Embodiment

FIG. 9 illustrates a gas decomposition apparatus 10 according to afourth embodiment of the present invention. In this gas decompositionapparatus 10, the electrolyte layer 15 is constituted by Nafion, whichis a PFC polymer 5 having a proton conductivity, and a biaxiallystretched porous PTFE membrane 3 configured to mechanically reinforcethe Nafion. The catalytic electrode layer 6 configured to decomposegases are constituted by a porous metal sheet 6 m entirely formed byplating and fine catalytic particles 11 carried by the porous metalsheet 6 m. It is not necessary to form the porous metal sheet 6 m usinga material other than the material of the gas-diffusion layer 8 in thefirst embodiment. The porous metal sheet 6 m may be formed of the samematerial as that of the gas-diffusion layer 8. The fine catalyticparticles 11 may be carried by other metal powder 21 m. The counterelectrode layer 7 is formed of porous carbon. Although the counterelectrode layer 7 does not carry fine catalytic particles, the counterelectrode layer 7 may carry fine catalytic particles. Since a reductionreaction occurs in the counter electrode layer (cathode) 7, even when anoncovalent carbon material is used for the counter electrode layer 7,there is no possibility that carbon monoxide is generated. Accordingly,due to a reason described below, porous carbon is preferably used.

As described above, the porous metal sheet 6 m entirely formed byplating can be made to have a large pore size and a large porosity.Thus, the porous metal sheet 6 m of the catalytic electrode layer 6 inFIG. 9 can make gas flow turbulent so that separation of gas from thesurface layer is always repeated and new gas is supplied to the surfacelayer. Accordingly, an increase in the applied voltage is appropriatelyreflected to promote a decomposition reaction. As a result, while theapplied voltage is increased, a decomposition reaction can be promotedwithout the possibility of the generation of carbon monoxide. Inparticular, in gas decomposition apparatuses used for deodorization ofliving spaces, the concentration of odorous gases is not high and hencethe reaction tends to occur at a low frequency. In a laminar flowgenerated along a planar surface, there is a portion where the flowstagnates is generated in the surface layer. Such a stagnant portion ismainly constituted by non-reactive gases such as air. As a result, mostodorous gas components at low concentrations pass without being broughtinto contact with the surface of the catalytic electrode layer 6. Thisis basically different from the case of fuel cells, which are configuredto decompose large amounts of gas components at high concentrations. Theporous metal sheet 6 m entirely formed by plating is a three-dimensionalnetwork metal material having continuous pores. By decreasing the widthof the frame portions of the metal material, the size of the pores canbe increased. Furthermore, for example, the porosity and therelationship between specific surface and pore size can be controlledwithin wide ranges. Accordingly, by adjusting the pore size, theporosity, and the specific surface, a flow in which a portion in contactwith the surface is always separated from the surface and new gas ismade to be brought into contact with the surface can be formed. Thus,odorous gas components at low concentrations can be efficientlydecomposed in a short period of time. Furthermore, the porosity can bemade large and hence gases can be made to flow smoothly. As for theporous metal sheet 6 m that is entirely formed by plating and can bemade to have large values in terms of specific surface, porosity, andthe like due to a large pore size and narrow frame portions, theabove-described CELMET (registered trademark) is preferably used. CELMET is produced by steps sequentially performed as follows: a foamingtreatment of a resin, a joining pore-formation treatment, electroless Niplating, Ni electroplating, and removal of the resin. In the presentembodiment, the porous metal material is entirely formed by plating.However, the porous metal material is not necessarily entirely formed byplating and may be partially formed by plating.

The catalytic particles 11 (21 m) can be anchored (carried) on theporous metal sheet 6 m by applying and drying a binder resin as in thefirst embodiment.

As illustrated in FIG. 9, the electrolyte layer 15 is constituted by thestretched porous PTFE membrane 3 and the PFC polymer 5 that fills gapsin the stretched porous PTFE membrane 3 and is in direct contact withthe two electrodes 6 and 7. The PFC polymer 5 loses its ion conductivityunless it is wet. However, the PFC polymer 5 being wet has considerablylow strength and is fragile. In particular, only repeating of drying andwetting of the PFC polymer 5 due to repeating of use and disuse candamage the PFC polymer 5. The stretched porous PTFE membrane 3 canreinforce the PFC polymer 5 to enhance the durability of the PFC polymer5. In addition, the stretched porous PTFE membrane 3, which is porous,negligibly affects the ion conductivity.

The stretched porous PTFE 3 illustrated in FIG. 9 is not covered with ahydrophilic resin; however, the stretched porous PTFE 3 may be coveredwith a hydrophilic resin. By forming a hydrophilic resin film, a largenumber of micro-nodes and fibers running through the micro-nodes and thePFC polymer are further integrated, that is, the contact resistancebetween the PTFE and the PFC polymer is increased. Thus, the reinforcingeffect provided by the PTFE is enhanced.

As for the stretched porous PTFE membrane 3, for example, POREFLON(registered trademark) manufactured by Sumitomo Electric Fine Polymer,Inc. is preferably used. In general, POREFLONs that are uniaxially andbiaxially stretched (standard: biaxially stretched) and have a pore sizeof 0.2 to 1 μM (standard: 0.2 μm), a thickness of 10 to 25 μm (standard:20 μm), and a porosity of 60% (standard: 70%) are preferably used. Amongthese POREFLONs, POREFLONs having a pore size of about 30 nm (0.03 μm)can be made to have a fine pore size and a small thickness of 1 μm orless and further 0.1 μm or less. Thus, such POREFLONs are considerablyadvantageous for reducing the thickness of the electrolyte layer 15. Byreducing the thickness of the electrolyte layer 15, the followingsignificant advantages can be provided.

(1) Water generated in the counter electrode layer (cathode) 7 isreadily distributed in the entirety of the PFC polymer 5. As describedabove, the PFC polymer 5 cannot exhibit proton conductivity withoutwater. When the electrolyte layer 15 is thin, water generated at theinterface between the counter electrode layer (cathode) 7 and theelectrolyte layer 15 can readily moisten even the interface between thecatalytic electrode layer (anode) 6 and the electrolyte layer 15.(2) The electrolyte accounts for a large portion of the internalresistance in the entire equivalent electric circuit. By reducing thethickness of the electrolyte, the current value can be increased, whichis advantageous for promoting a decomposition reaction.(3) A sufficiently high strength is also provided. The above-describedPOREFLON having a pore size of 30 nm has a thickness of 1 to 2 μm. Whenthis POREFLON is used in the state of being bonded to a reinforcing filmhaving a thickness of about 10 μm, in spite of the small thickness,sufficient reinforcement is achieved in terms of strength.

The gas decomposition apparatus 10 illustrated in FIG. 9 is intended todecompose odorous gases being mixed with air, such as ethanol, methanol,acetaldehyde, and formaldehyde. Thus, a potential is applied to thecatalytic electrode layer (anode) 6 and the counter electrode layer(cathode) 7 such that odorous gases are decomposed by an oxidationreaction at the catalytic electrode layer 6. Specifically, protons arefed from the catalytic electrode layer 6 to the PFC polymer 5 of theelectrolyte layer 15 to release electrons to external wiring. At thistime, air containing odorous gases is introduced with a pump (not shown)into the catalytic electrode layer (anode) 6 and air (cleaned air)containing gases provided by decomposition through the anode reaction isdischarged from the outlet to the ambient environment. At the counterelectrode layer 7, protons having been conducted through the PFC polymer5 of the electrolyte layer 15 cause a reduction reaction with air andelectrons supplied from the wiring to the counter electrode layer(cathode) 7 to generate water. To supply air to the counter electrodelayer 7, air is introduced to the counter electrode layer 7 from theoutside. As described above, water generated at the counter electrodelayer (cathode) 7 can moisten the PFC polymer 5.

To effectively distribute, in the electrolyte layer 15, water generatedby the water generation reaction at the counter electrode layer 7, asdescribed above, the electrolyte layer 15 preferably has a smallthickness. In particular, in gas decomposition apparatuses used fordeodorization of living spaces, the concentration of odorous gases isnot high and hence the reaction tends to occur at a low frequency andthe amount of water generated is small. Thus, reduction in the thicknessof the electrolyte layer 15 or the PFC polymer 5 is an important factor.Furthermore, as described above, the electrolyte layer 15 is defined aselectrical resistance in the gas decomposition apparatus 10. By reducingthe thickness of the electrolyte layer 15, the electrical resistance isdecreased. Accordingly, ion conductivity can be made high and theefficiency of gas decomposition can be enhanced.

The electrolyte layer 15 in the gas decomposition apparatus 10 accordingto the present embodiment can be produced by preparing and immersing thestretched porous PTFE membrane 3 so as to have a predetermined thicknessin a solution having been obtained by dissolving a PFC polymer in asolvent, removing the solvent, and drying the membrane. At this time, toestablish an electric contact between the catalytic electrode layeranode 6 and the counter electrode layer (cathode) 7, the PFC polymer 5are made to be exposed on the front and back surfaces of the electrolytelayer 15. As described above, as for the catalytic electrode layer(anode) 6, metal powder 21 m carrying the fine catalytic particles 11are preferably dispersed in and held by the porous metal sheet 6 m whileconductive contact is ensured. The counter electrode layer (cathode) 7will be described below. The catalytic electrode layer (anode) 6 and thecounter electrode layer (cathode) 7 are placed so as to sandwich theresultant electrolyte layer 15 from the front and back surfaces of theelectrolyte layer 15. The catalytic electrode layer (anode) 6, thecounter electrode layer (cathode) 7, and the electrolyte layer 15 arethen heated at about 120° C. and bonded together by hot pressing to forma membrane electrode assembly (MEA), which is a stack. Other than such aproduction method, various modified production methods may be employed:for example, a production method in which the electrolyte layer 15 isstacked on the electrodes layers 6 and 7.

As described above, in the counter electrode layer (cathode) 7, areduction reaction occurs and carbon is not oxidized. Thus, a laminarporous material that is conductive and can hold conductive particlesbeing dispersed therein is preferably used for the counter electrodelayer (cathode) 7. For example, a porous sheet constituted by carbonfibers such as carbon paper or carbon felt is preferably used. Inparticular, a porous sheet constituted by carbon fibers has excellentresistance to strongly acidic atmosphere due to protons generated by adecomposition reaction. In addition, such a porous sheet is porous andhence can carry a large number of the fine catalytic particles 11 (metalpowder 21 m). Thus, the efficiency of decomposing odorous gases can befurther enhanced, which is advantageous.

For example, the carbon paper may be produced by subjecting carbonfibers in the form of monofilaments to wet or dry paper making or thelike so as to have any thickness or any basis weight. The carbon felt isproduced by, for example, subjecting carbon fibers in the form ofmonofilaments to carding or the like, stacking on one another, andbonding together by a needle punching or the like. A carbon felt havingany mean fiber size and any mass per unit area can be used. However, toreduce the thickness of a gas decomposition apparatus as much aspossible, carbon paper is preferably used as a base.

As for the counter electrode layer (cathode) 7 in which fine catalyticparticles are dispersed in and held by a conductive base such as carbonpaper described above, cathodes having various structures may beemployed. Specifically, as for the counter electrode layer (cathode) 7,(1) a structure in which fine catalytic particles are directly carriedon the surfaces of a conductive base may be employed; or (2) compositeparticles in which the fine catalytic particles are carried on thesurfaces of a conductive powder of carbon black or the like aredispersed in a binder resin having proton conductivity to provide amembrane and the membrane may be stacked on the surfaces of a conductivebase. Such a catalytic electrode layer of (1) described above isprepared by, for example, in the state in which a conductive base isimmersed in a solution containing metal ions serving as the source offine catalytic particles, reducing the metal ions by the action of areducing agent so that the metal ions are precipitated in the form offine particles and directly carried on the surfaces of the conductivebase (in the case of a porous conductive base, the surfaces include theinner surfaces of pores).

Fifth Embodiment

FIG. 10 is a schematic view of an MEA structure of a gas decompositionapparatus 10 according to a fifth embodiment of the present invention.FIG. 11 specifically illustrates the gas decomposition apparatus 10having this MEA structure. In the MEA structure of the gas decompositionapparatus 10, a catalytic electrode layer 6 and a counter electrodelayer 7 are disposed so as to sandwich an electrolyte layer (solidelectrolyte) 15 that is a polymeric membrane containing an ionic liquid45, the polymeric membrane being formed by dissolving and polymerizingvinyl monomers or the like serving as a resin material in an ionicliquid having high ion conductivity. Both the catalytic electrode layer6 and the counter electrode layer 7 contain fine catalytic particles ofplatinum or the like. The fine catalytic particles may be contained inthe two electrodes in the form of being carried by a conductive powder(carriers) or may be, without carriers, directly made to adhere to (becarried by) electrode sheets by plating or the like, the electrodesheets being conductive bases for forming the electrodes. A porousgas-diffusion layer 8 through which an odorous gas to be decomposed isintroduced and from which the odorous gas having been subjected to adecomposition reaction (anode reaction or oxidation reaction) isdischarged is provided for the catalytic electrode layer 6. The porousgas-diffusion layer 8 is preferably composed of a conductive materialsuch as CELMET (registered trademark), which is a porous metalmanufactured by Sumitomo Electric Industries, Ltd. A porousgas-diffusion layer 9 composed of CELMET or the like is also providedfor the counter electrode layer 7 for the purpose of introducing air toprovide oxygen for a cathode reaction and discharging water generated bythe cathode reaction (reduction reaction).

The gas decomposition apparatus 10 has a feature that the electrolytelayer 15 is formed using the ionic liquid 45. Since ionic liquids havehigh ion conductivity, unlike existing gas decomposition apparatuses,stable molecules of toluene, benzene, and the like can be efficientlydecomposed by applying a voltage equal to or higher than thedecomposition voltage of water. The reason for this is as follows.Aromatic compounds such as toluene and benzene have higher decompositionvoltages Vd than water. Ionic liquids are stable within and outside ofthe potential window corresponding to the decomposition voltages on theoxidation side and the reduction side. Accordingly, even when a voltageequal to or higher than Vd is applied between the anode and the cathodeof the gas decomposition apparatus, toluene and benzene can bedecomposed while a stable state is maintained. Ionic liquids arepreferably in the form of solid thin membranes.

The application of ionic liquids to electrolytes of lithium cells, fuelcells, and the like has been studied. However, studies on theapplication of ionic liquids to gas decomposition apparatuses having MEAstructures are unheard of This is probably because, as described above(refer to FIG. 3), by using a PFC polymer (Nafion), even when a voltagehigher than the decomposition voltage of water is applied between ananode and a cathode, the decomposition of odorous gases is seeminglyperformed without problems. When PFC polymers that have been proposed todate are used as electrolytes, odorous gases can be decomposed withoutproblems under the application of a high voltage of about 2 V. However,use of PFC polymers as electrolytes, for example, in a long-termoperation or a dry environment, probably causes the following problems.That is, the application of a voltage higher than the decompositionvoltage of water results in a wasteful energy consumption with regard tothe decomposition of water contained in PFC polymers; and a decrease inthe gas decomposition rate due to use of electrical energy for thedecomposition of water, the electrical energy being supposed to be usedfor the decomposition of gases.

The above-described problems cannot be found by simply producing orusing gas decomposition apparatuses employing PFC polymers. This isbecause, as illustrated in FIG. 3, in use of PFC polymers for gasdecomposition apparatuses, even when a voltage of about 2 V is appliedbetween the anode and the cathode, seemingly normal gas decompositionappropriate to the applied voltage proceeds. However, the inventors ofthe present invention have studied the principle in which PFC polymersexhibit ion conductivity and the possibility of problems occurring incontinuous use for a long period of time. As a result, the inventorshave conceived the above-described application of ionic liquids toelectrolytes. Hereinafter, components of the gas decomposition apparatus10 in FIGS. 10 and 11 will be described.

(1) Electrolyte Layer 15

The ionic liquid 45 included in the electrolyte layer 15 is a salt alsoreferred to as a low-temperature molten salt or a room-temperaturemolten salt. Ionic liquids are not clearly defined; however, ionicliquids generally refer to liquid salts that have substantially zerovapor pressure, are fire retardant and ionic, but have low viscosity andhigh decomposition voltage. Examples of typical ionic liquids are listedbelow in the form of being divided into cations and anions. However,another ionic liquid other than those listed below may be used.

Cation: one or more selected from trimethylpropylammonium (TMPA),trimethylmethoxymethylammonium (TMMMA), trimethylphenylammonium (TMPhA),trimethylhexylammonium (TMHA), 1-ethyl-3-methylimidazolium (EMI),triethylsulfonium (TES), butylpyridinium (BP), 1-butyl-3-methylimidazole(BMI), and the like.

Anion: one or more selected from trifluoromethanesulfonylimide (TFSI),fluorosulfonylimide (FSI), trifluorosulfonylacetylimide (TSAC),trifluoromethanesulfonylmethyl (TFSM), trifluoromethanesulfate (TfO),AlCl₄ (chloroaluminate), BF₄ (tetrafluoroborate), PF₆(hexafluorophosphate), F (fluoride), Cl (chloride), I (iodide), Br(bromide), and the like.

By using readily available ionic liquids in which the above-listedcations and anions are combined, electrolytes having low melting points,high decomposition voltages (stable in terms of voltage), and high ionconductivity can be obtained. For example, EMI+TFSI− is reduced at −2.1V with respect to I+/I− (+1.1 V with respect to Li+/Li−) and is verystable. When EMI+TFSI− and TMHA+Tf2N− are measured in terms of cyclicvoltammogram, the potential window extends from about −3.0 to +2.0 V,which is very large and stable compared with the potential window interms of water. Accordingly, a gas decomposition apparatus employing anionic liquid as an electrolyte can function with stability even when,for example, a voltage slightly exceeding 2.5 V, which is thedecomposition voltage of toluene, is applied. Naturally, ethanol havinga decomposition voltage of 1.3 V, acetaldehyde having a decompositionvoltage of 1.2 V, and the like can be decomposed without problems.

(2) Gelatinization of Ionic Liquid

The electrolyte layer 15 in FIGS. 10 and 11 is a polymeric membranecontaining an ionic liquid and has ion conductivity. Such a polymericmembrane containing an ionic liquid can be obtained by, for example,dissolving vinyl monomers in an ionic liquid and causing radicalpolymerization to proceed in the ionic liquid. At this time, the ionicliquid is contained in the network of the polymer and the state in whichthe ion conductivity of the ionic liquid is held in the network isobtained. This is a solid membrane obtained by gelatinizing the ionicliquid. Such a networked polymer may be synthesized by adding divinylmonomers to vinyl monomers. Such vinyl monomers and divinyl monomers arenot limitative and any polymer may be employed as long as an ionicliquid is contained in a networked polymer by polymerization and the ionconductivity of the ionic liquid can be held in the polymer.

(3) Electrodes and Fine Catalytic Particles

As for a conductive base included in the catalytic electrode layer 6 orthe counter electrode layer 7 containing a catalyst, a laminar base thathas conductivity and can carry fine catalytic particles thereon can beused. For example, a porous sheet constituted by metal fiber obtained byprocessing nickel fine wires, niobium fine wires, or the like into afibrous form; a porous metal material such as CELMET (a metal madeporous by casting); a metal sinter; or carbon fiber such as carbon paperor carbon felt can be used. Since such a porous sheet is porous, finecatalytic particles can be directly carried on the porous sheet. Thus,it is not necessary to prepare carriers such as a powder. The term“powder” is a name that identifies the classification of a product.However, other than such a classification name for products, the term“powder” also refers to particles having a considerably larger size thanfine catalytic particles.

As described below, such a porous sheet constituted by metal fiber,CELMET, or the like is preferred for preventing the generation of carbonmonoxide when gas decomposition is performed at a temperature higherthan room temperature by applying a voltage of 1.5 V or higher betweenthe catalytic electrode and the counter electrode. However, when thetemperature is not made higher than room temperature and a voltage ofless than 1.5 V is applied between the two electrodes, there is nopossibility that carbon monoxide is generated. In such cases, a poroussheet constituted by carbon fiber such as noncovalent carbon paper,carbon felt, or the like may be used. Such a porous sheet constituted bycarbon fiber has excellent resistance to strongly acidic atmospherecontaining protons generated by a gas decomposition reaction.

A conductive base containing fine catalytic particles may employ thefollowing structures. (1) As illustrated in FIG. 12, fine catalyticparticles 11 are directly carried by the surface of the above-describedporous conductive base 6 d. The conductive base 6 d may be metal fiber,CELMET, or the like, or carbon fiber.

(2) As illustrated in FIG. 13, the structure in which a metal powder(carriers) 21 m of nickel, cobalt, silver, molybdenum, or the like onthe surfaces of which the fine catalytic particles 11 are carried aredispersed in a binder resin having proton permeability and provided onthe surface of the conductive base 6 d can be employed. As for thepowder, other than the above-described metal powder, a conductive carbonpowder 21 d of carbon black, acetylene black, or the like may be used.The conductive carbon powder 21 d may be used as carriers; and thecarriers may be made to carry the fine catalytic particles 11, anddispersed in the binder resin, and provided on the surface of theconductive base 6 d.

As for the catalytic electrode layer 6 in (1) above (although “thecounter electrode layer 7 containing a catalyst” is also included, thedescription thereof is omitted for the sake of simplicity), for example,in the state in which the conductive base 6 d is immersed in a solutioncontaining metal ions forming the fine catalytic particles 11, the metalions are reduced with a reducing agent to precipitate the fine catalyticparticles 11 composed of the metal onto the conductive base 6 d. Asdescribed above, when a porous sheet is used as the conductive base, thefine catalytic particles also precipitate onto the inner surfaces ofmany pores. In such precipitation, the fine catalytic particles arecarried by the conductive base.

A catalytic electrode layer in (2) above is formed as follows. Forexample, a metal powder 21 m of nickel, silver, or the like or aconductive carbon powder 21 d of carbon black or the like is prepared.In the same manner as described above, such a powder 21 m, 21 d is thenimmersed in a solution containing metal ions forming fine catalyticparticles and the metal is precipitated in the form of fine particlesonto the surfaces of the conductive powder by using a reducing agent.The catalyst-carrying powder 21 m, 21 d, 11 is mixed with a solution ofa binder resin having ion permeability to prepare a coating solution.The coating solution is then applied to a surface of the conductive baseand dried to form a binder resin membrane in which the carrier powder isdispersed. In the catalytic electrode layer of (2) above, as describedabove, porous metal fiber such as nickel fiber or niobium fiber; aporous metal material such as CELMET; a metal sinter; carbon fiber suchas carbon paper; or the like is used as the conductive base. Inaddition, the binder resin membrane is stacked so as to be in contactwith the electrolyte.

In such a stack, while the contact between the fine catalytic particlesand odorous components is maintained with the porous conductive base,the catalyst-carrying powder is dispersed in a membrane composed of abinder resin having proton permeability, and the membrane is sandwichedbetween the conductive base and the solid electrolyte. Accordingly, forexample, removal of the fine catalytic particles is suppressed and hencethe catalytic action can be maintained for a longer period of time.

As for the fine catalytic particles, a rare metal such as platinum,ruthenium, palladium, iridium, or osmium; an iron group metal such asiron, cobalt, or nickel; or a noble metal such as vanadium, manganese,silver, or gold is preferably used. Alternatively, to enhance a specialfunction, fine catalytic particles composed of an alloy of such metalsmay be used. For example, to enhance a catalyst-poison resistanceserving as a catalytic function, an alloy in which the mass ratio ofplatinum to palladium Pt/Pd satisfies about 7/3 to 9/1 may be used.

In the gas decomposition apparatus 10, by applying a voltage of, forexample, about 1.5 V to the two electrodes at room temperature, odorousgases such as acetaldehyde, ethanol, and toluene can be rapidlydecomposed at a high energy efficiency. For example, when theelectrolyte includes a PFC polymer such as Nafion, the application of avoltage of 1.5 V between the two electrodes causes the electrolysis ofwater. Thus, supplied energy is used for the electrolysis of water andhence the energy efficiency is degraded. In addition, since the water inNafion is decomposed, moisture necessarily required for ion conductionis decreased to less than the appropriate amount. Thus, the ionconductivity is degraded and the possibility of the termination of gasdecomposition is caused. As in a gas decomposition apparatus accordingto the present embodiment, by using an ionic liquid for an electrolyte,a stable state can be maintained under the application of voltage. Inaddition, by using a polymeric membrane containing an ionic liquid,since the ionic liquid can be handled as a solid electrolyte membrane,the production of such a gas decomposition apparatus can be simplified.

Sixth Embodiment

The structure of a gas decomposition apparatus according to a sixthembodiment of the present invention is the same as that illustrated inFIGS. 10 and 11. Use of the ionic liquid 45 for the electrolyte layer 15is also the same. The feature of the gas decomposition apparatus 10 ofthe present embodiment is that, to prevent the possibility of thegeneration of carbon monoxide even when a voltage of higher than 1.5 Vis applied between the catalytic electrode layer 6 and the counterelectrode layer 7, noncovalent carbon materials are not disposed asconductive materials in contact with the fine catalytic particles 11.The gas decomposition apparatus 10 according to the fifth embodiment isthe same as in the present embodiment as long as a metal is used for theconductive base of the catalytic electrode layer 6 or a metal is usedfor the catalyst carrier powder. However, the present embodiment isdifferent from the fifth embodiment in that, to provide a configurationin which the possibility of the generation of carbon monoxide isprevented with certainty, configurations in which fine catalyticparticles are directly carried by noncovalent carbon fiber or the likeare excluded.

FIG. 2 described above is a graph illustrating experimental resultsserving as the reason why a gas decomposition apparatus according to thepresent embodiment is provided. FIG. 2 illustrates experimental resultsof a gas decomposition apparatus in which platinum was used for finecatalytic particles, carbon black was used for catalyst carriers, carbonpaper was used for the conductive base of a catalytic electrode, andNafion (PFC polymer) was used for an electrolyte. Both the carbon blackof the catalyst carriers and the carbon paper of the conductive base arenoncovalent. FIG. 2 shows that the application of a voltage of 1.5 Vbetween the two electrodes at 80° C. results in the generation of carbonmonoxide. In addition, the application of a voltage of 2 V at roomtemperature also results in the generation of carbon monoxide. Sourcesof the generation of carbon monoxide other than the air are theabove-described carbon materials. Decomposition of gases that are notlimited to hydrocarbons occurs at a position where the fine catalyticparticles 11, the catalytic electrode layer 6 (catalyst carriers 21 inthe case of using catalyst carriers), and the electrolyte layer 15 arein contact with each other. Carbon monoxide is generated whennoncovalent carbon black or noncovalent carbon paper is present in sucha contact position.

FIG. 3 described above, which is not directly related to the generationof carbon monoxide, is a graph illustrating the influence of the voltagebetween the anode and the cathode on the decomposition rate ofacetaldehyde decomposed with the gas decomposition apparatus describedin conjunction with FIG. 2. FIG. 3 indicates that, as the voltagebetween the two electrodes increases from 1 V to 1.5 V to 2 V, theconcentration of acetaldehyde decreases in a shorter time. As describedabove, this gas decomposition apparatus contained Nafion; however, thegas decomposition process was seemingly performed without problems underthe application of a voltage of 2 V. In spite of such a gasdecomposition process, the inventor of the present invention hasconcluded that, when an aromatic compound gas is decomposed, use of anionic liquid for an electrolyte is advantageous in continuous use for along period of time or in dry environments.

(Catalytic Electrode and Fine Catalytic Particles in the PresentEmbodiment)

As for a conductive base included in the catalytic electrode layer, alaminar base that has conductivity and can carry fine catalyticparticles thereon can be used. For example, as illustrated in FIG. 14(a), a porous sheet constituted by metal fibers such as nickel fine wiresor niobium fine wires or a porous metal material sheet 6 m such asCELMET is preferred. Such a porous sheet constituted by metal fibers,the porous metal material sheet 6 m, or the like is preferred in view ofprevention of the generation of carbon monoxide caused by theabove-described mechanism.

As illustrated in FIG. 14( b), in spite of employing noncovalentconductive base 6 g of carbon fiber as frames, a conductive baseincluding a surface layer of the conductive diamond 30 containing animpurity such as boron at a high concentration can be used withoutproblems. In the conductive diamond 30, since carbon atoms are bondedthrough covalent bonds, the conductive diamond 30 not only has highstrength but also has high resistance to external voltage and is verystable against external voltage compared with noncovalent carbon black,graphite, and the like. The frames on which a thin film of theconductive diamond 30 is formed may be, for example, the above-describedporous metal sheet 6 m or the porous conductive base 6 g of carbon fibersuch as carbon paper or carbon felt. Even after a thin film of theconductive diamond 30 is formed on the porous frames, the resultantmaterial is desirably still porous. In the above-described cases, thefine catalytic particles are made not to be in direct contact withnoncovalent carbon materials such as carbon paper and carbon felt.

A conductive base including fine catalytic particles may employ thefollowing structures. (1) The fine catalytic particles are directlycarried on a surface of the conductive base. The surface of theconductive base may be a layer formed of conductive diamond or may bemetal fiber or the like of the conductive base.

(2) As illustrated in FIG. 15( a), a structure in which a metal powder(carriers or nugget-shaped carriers) 21 m of nickel, cobalt, silver,molybdenum, or the like on the surface of which the fine catalyticparticles 11 are carried are dispersed in a binder resin having protonpermeability and provided on a surface of the conductive base can beemployed. Alternatively, as illustrated in FIG. 15( b), other than theabove-described metal powder, a conductive carbon powder 21 d of carbonblack, acetylene black, or the like may be used. In this case, the thinfilm(s) of the conductive diamond 30 must be formed on the surfaces ofthe conductive carbon powder 21 d. Such (conductive carbon powder21/surface layer of the conductive diamond 30) may be used as carriers;and the carriers may be made to carry fine catalytic particles,dispersed in the binder resin, and provided on the surface of theconductive base. Core parts of the carriers such as conductive carbonpowder 21 d or the like in FIG. 15( b) may be an insulating powder andthe electrical connection between the carriers and the catalyticelectrode layer 6 can be established through films of the conductivediamond 30.

As for the catalytic electrode layer 6 in (1) above, for example, in thestate in which the conductive base is, immersed in a solution containingmetal ions forming the fine catalytic particles, the metal ions arereduced with a reducing agent to precipitate the fine catalyticparticles composed of the metal onto the conductive base. As describedabove, when a porous sheet is used as the conductive base, the finecatalytic particles also precipitate onto the inner surfaces of manypores. In such precipitation, the fine catalytic particles are carriedby the conductive base.

A catalytic electrode layer in (2) above is formed as follows. Forexample, (i) a metal powder of nickel, silver, or the like, (ii) apowder in which conductive-diamond surface layers are formed on a metalpowder, (iii) a composite carbon powder in which conductive-diamond thinfilms are formed on the surfaces of a conductive carbon powder such ascarbon black, or (iv) a powder in which conductive-diamond thin filmsare formed on an insulating powder is prepared. In the same manner asdescribed above, such a conductive powder is then immersed in a solutioncontaining metal ions forming fine catalytic particles and the metal isprecipitated in the form of fine particles onto the surfaces of theconductive powder by using a reducing agent. The catalyst-carryingpowder is mixed with a solution of a binder resin having ionpermeability to prepare a coating solution. The coating solution is thenapplied to the surface of the conductive base and dried to form a binderresin membrane in which the carrier powder is dispersed. In thecatalytic electrode layer of (2) above, as described above, porous metalfiber or the like such as nickel fiber or niobium fiber, a porousmaterial obtained by coating such metal fiber or the like withconductive diamond, a porous material obtained by coating carbon fibersuch as carbon paper with a conductive-diamond thin film, or the like isused as the conductive base. In addition, the membrane of the binderresin is stacked so as to be in contact with the electrolyte.

In such a stack, while the contact between the fine catalytic particlesand odorous components is maintained with the porous conductive base,the catalyst-carrying powder is dispersed in a membrane composed of abinder resin having proton permeability, and the membrane is sandwichedbetween the conductive base and the solid electrolyte. Accordingly, forexample, removal of the fine catalytic particles is suppressed and hencethe catalytic action can be maintained for a longer period of time.

In a gas decomposition apparatus according to the present embodiment,the electrolyte is formed using an ionic liquid. Thus, gas decompositioncan be performed while a voltage equal to or higher than thedecomposition voltage of water is applied to the two electrodes. At thistime, the possibility of the generation of carbon monoxide can beprevented. Therefore, odorous gases of many types including toluene,ethanol, and the like can be rapidly decomposed with a high energyefficiency while safety is ensured.

Instead of an ionic liquid in the fifth and sixth embodiments, CsHSO₄, amolten salt, or a solid oxide electrolyte functioning under heating maybe used. Thus, the scope of selection of the electrolyte can be expandedin accordance with, for example, the operation environment, requiredperformance, or required cost effectiveness of the gas decompositionapparatus. For example, since CsHSO₄ can function at a low temperatureof about 100° C., CsHSO₄ is suitable for applications in which costeffectiveness and high decomposition capability are required. Ionicliquids are suitable for applications in which small size, low power,and the like are of higher priority than cost effectiveness. Solid oxideelectrolytes, which need to be heated to high temperature of 300° C. orhigher, are suitable for applications in which high decompositioncapability, durability, being field-proven, cost effectiveness, and thelike are of high priority.

Seventh Embodiment

The configuration of a gas decomposition apparatus according to aseventh embodiment of the present invention is the same as that of thegas decomposition apparatus 10 illustrated in FIG. 1 . . . . However,the present embodiment is unique with regard to the material of aportion of an electrode, the portion being in contact with theelectrolyte. The present invention has a feature that a portion of adecomposition-side electrode layer 6, the portion being in contact withthe electrolyte, is composed of a material that is inert to water. FIG.16 illustrates voltage-current curves obtained when a standard hydrogenelectrode (SHE) is defined as zero potential, an electrode beingimmersed in water is used as a working electrode, and potential isapplied to the working electrode. When a platinum electrode is used asthe working electrode (a broken line in FIG. 16), in the positiveregion, the current density exponentially increases beyond about +0.5 V;and in the negative region, the current density exponentially increasesin the opposite direction beyond −0.3 V toward negative potential.Current substantially does not flow between the current rising potentialin the positive region and the current rising potential in the negativeregion. Such a potential range or a potential width Vw where currentsubstantially does not flow is referred to as a window. For water, Vw is1.23 V. The potential width Vw of the window depends on the material(water in the present case) into which the electrode is immersed. As forthe material of the electrode, the potential width Vw of the window doesnot vary as long as electrodes composed of the platinum group and thelike having high catalytic activity are used. In the positive region, asthe current increases, oxygen is generated at the working electrode; andin the negative region, as the current increases, hydrogen is generated.

In the above-described measurement of potential with water or an aqueoussolution, the reference electrode is preferably a silver/silver chlorideelectrode (Ag/AgCl electrode), mercury/mercurous chloride electrode(calomel electrode, Hg/Hg₂Cl₂ electrode), a hydrogen electrode, or thelike. As for the working electrode (work electrode), an appropriateelectrode material considerably varies; however, in the case of water,platinum (Pt), carbon (C), mercury (Hg), or the like is preferably used.Flow of current needs to be suppressed as much as possible. Accordingly,potentiometry in which measurement is performed in the state wherecurrent does not flow through the system is preferably used.Alternatively, it is preferred that an auxiliary electrode forundertaking current other than the reference electrode be prepared and athree-electrode potentiostat be used. In addition, since there are manyvariable factors (disturbance factors) in the measurement of potential,extreme caution must be taken.

However, when a material that does not have a high catalytic activity,for example, a material that is inert to water is used for the workingelectrode, the current rising potential in the positive region becomeshigh compared with a catalytic electrode having high catalytic activityand the current rising potential in the negative region shifts towardthe negative direction. Thus, in spite of use of the same water, thewindow extends. For example, a working electrode covered with conductivediamond has a window having a potential width of about 2.5 V. That is,when the working electrode is formed of a material that is inert towater, water is not decomposed and a stable state is maintained evenunder the application of a higher voltage. Accordingly, when a materialthat is inert to water is used for the decomposition-side electrode,even when an electrolyte containing water is used, odorous gases can bedecomposed under the application of a voltage of 1.23 V or higherbetween the decomposition-side electrode and the counter electrodewithout decomposing the water in the electrolyte. As for thedecomposition-side electrode, a wide window is preferred in view ofmaintaining water in a stable state. However, a further shift of thecurrent rising potential in the negative region toward the negativedirection is more preferred in view of decomposing odorous componentsand generating protons without decomposing water at thedecomposition-side electrode. According to the present invention, amaterial that is inert to water is used for a portion of thedecomposition-side electrode 6, the portion being in contact with theelectrolyte. Thus, even when the electrolyte contains water, odorous gascomponents can be decomposed without decomposing the water in theelectrolyte.

FIG. 17 illustrates an electrolyte-side portion of thedecomposition-side electrode layer 6 of a gas decomposition apparatusaccording to the seventh embodiment of the present invention. Thepresent embodiment has a feature that the decomposition-side electrodelayer 6 is prepared with a porous sheet 16 covered with a thin film ofthe conductive diamond 30. The core material of the porous sheet 16 ofthe decomposition-side electrode layer 6 may be any material such as ametal, an insulating material, a noncovalent carbon fiber, or the like;preferably, Si, Mo, Nb, a metal oxide ceramic, or the like. Theconductive diamond, which is crystals in which carbon atoms arecovalently bonded together, has been made to have conductivity by beingdoped with an impurity such as boron at a high concentration. Theconductive diamond is not restricted to covalently bonded carboncrystals and may be a material referred to as a diamond like material aslong as it contains an impurity element at a high concentration and haselectrical conductivity.

When the porous sheet 16 is formed of a metal, any porous metal such asmetal fiber obtained by processing nickel fine wires, niobium finewires, or the like into a fibrous form; a porous metal material such asCELMET (a metal made porous by casting); or a metal sinter obtained bysintering a metal powder may be used. The electrolyte layer 15 may be aPFC polymer, any aqueous electrolyte, or any nonaqueous electrolyte. Asfor such a nonaqueous electrolyte, for example, an ionic liquid may beused. Fine catalytic particles are not used.

When the porous sheet covered with the thin film(s) of the conductivediamond 30 is a fibrous sheet, it is preferred that the thin film(s) ofthe conductive diamond 30 be formed on threads and the threads be thenwoven together. In the case of a porous metal material such as CELMET,it is preferred that the porous metal material be placed in a plasma CVDchamber, irradiated with carbon plasma, and doped with a p-type impuritysuch as boron at a high concentration to form the thin film(s) of theconductive diamond 30 both on an outer surface and an inner surface ofthe porous material. A porous insulating material can be treated in thesame manner.

Eighth Embodiment

FIG. 18 illustrates an electrolyte-side portion of a decomposition-sideelectrode layer 6 of a gas decomposition apparatus according to aneighth embodiment of the present invention. The present embodiment has afeature that an inert powder 33 that is inert to water such as Ti₄O₇powder or PbO₂ powder is used for a porous conductive base. In FIG. 18,the porous decomposition-side electrode layer 6 is formed by sinteringthe inert powder 33. In the inert powder 33, there are gaps or pores 29.As long as the decomposition-side electrode layer 6 is porous and hasconductivity, the decomposition-side electrode layer 6 is notnecessarily formed from a single powder.

The Ti₄O₇ powder and the PbO₂ powder are well-known powder materials andcommercially available products thereof may be used. As for thetechnique of forming the decomposition-side electrode layer 6 from Ti₄O₇powder or PbO₂ powder, a well-known powder sintering technique can beused. The porosity in terms of many pores can be adjusted with pressureapplied upon pressing Ti₄O₇ powder or PbO₂ powder charged in a mold forthe decomposition-side electrode layer 6 and the particle size (particlediameter) of such a powder.

As for the inert powder 33, for example, Ti₄O₇ is one of compoundsreferred to as Magneli phases and exhibits metallic conductivity at roomtemperature due to contribution of delocalized 3 d electrons of titaniumto electrical conductivity. PbO₂ is a conductive material having beenused for positive plates of lead-acid batteries. Such Ti₄O₇ and PbO₂ areinert to water, expand the window in terms of water illustrated in FIG.16, and shift further the current rising potential in the negativeregion toward the negative direction.

Accordingly, in the state in which a voltage of the decompositionvoltage of water (1.23 V) or higher is applied between thedecomposition-side electrode and the counter electrode, water is notdecomposed. Thus, by applying such a high voltage, the decompositionrate of odorous components can be increased and odorous gases havinghigh decomposition voltages such as aromatic compounds can be decomposedwith a high energy efficiency. As a result, while a PFC polymerelectrolyte that necessarily requires water such as Nafion or an aqueouselectrolyte is used, lasting stability for the state in which theabove-described preferred advantages are exhibited under the applicationof a voltage equal to or higher than the decomposition voltage of watercan be achieved.

The electrolyte is not restricted to PFC polymers and may be anelectrolyte that does not require or contain water. For example, theelectrolyte may be (1) a room-temperature molten salt or an ionic liquidfunctioning at room temperature or (2) CsHSO₄, a phosphoric acid-basedproton conductor such as (NH₄)₂K_(1-x)PO₃, a molten salt, or a solidoxide electrolyte functioning under heating. Alternatively, phosphoricacid may be used as the electrolyte. Thus, the scope of selection of theelectrolyte can be expanded in accordance with, for example, theoperation environment, required performance, or required costeffectiveness of the gas decomposition apparatus. For example, sinceCsHSO₄ can function at a low temperature of about 100° C., CsHSO₄ issuitable for applications in which cost effectiveness and highdecomposition capability are required. Ionic liquids are suitable forapplications in which small size, low power, and the like are of higherpriority than cost effectiveness. Solid oxide electrolytes, which needto be heated to a high temperature of 300° C. or higher, are suitablefor applications in which high decomposition capability, durability,being field-proven, cost effectiveness, and the like are of highpriority.

Ninth Embodiment

FIG. 19 illustrates an electrolyte-side portion of a decomposition-sideelectrode layer 6 of a gas decomposition apparatus according to a ninthembodiment of the present invention. The present embodiment has afeature that a porous sheet covered with conductive diamond 30illustrated in FIG. 17 is used and an inert powder 33 that is inert towater such as Ti₄O₇ powder or PbO₂ is made to adhere to the poroussheet. When the decomposition-side electrode layer 6 has the structureillustrated in FIG. 17, the surface area of regions where odorous gasescan be decomposed is increased and projected portions (inert powder 33itself) that are highly frequently in contact with odorous gases can beformed.

The decomposition-side electrode layer 6 is preferably produced bydispersing the inert powder 33 in a binder resin having ion permeabilityand placing the inert powder 33 on a porous sheet covered with theconductive diamond 30. In this case, the inert powder 33 is mixed with asolution of the binder resin having ion permeability to prepare acoating solution. The coating solution is then applied to a surface ofthe porous sheet covered with the conductive diamond and dried. Thus, abinder resin membrane in which the inert powder 33 is dispersed isformed. As for the porous sheet to be covered with conductive diamond,as described above, porous metal fiber such as nickel fiber or niobiumfiber; a porous metal material such as CELMET; a metal sinter; carbonfiber such as carbon paper; insulating fiber; or the like may be used.The binder resin membrane is stacked so as to be in contact with theelectrolyte.

In the above-described configuration of the decomposition-side electrodelayer 6, when a voltage equal to or higher than the decompositionvoltage of water is applied between the decomposition-side electrodelayer 6 and the counter electrode layer 7, since the inert powder 33projects from the surface of the base, the decomposition of odorousgases can be promoted on the surface of the inert powder 33 by (1) anincrease in the surface area, (2) an increase in the frequency of thecontact with odorous gases, and (3) other unknown reasons. As a result,while a perfluorocarbon polymer electrolyte that necessarily requireswater such as Nafion or an aqueous electrolyte is used, lastingstability for the state in which odorous components of many types can bedecomposed at a high rate under the application of a voltage equal to orhigher than the decomposition voltage of water can be achieved.

(Regarding Application of Voltage and Potential)

As illustrated in FIG. 9, the application of the voltage V of thevoltage source to the gas decomposition apparatus 10 does not result inthe application of the voltage V itself between the catalytic electrodelayer anode 6 and the counter electrode layer (cathode) 7. This has beenrepeatedly described. When the (catalytic electrode layer anode6/electrolyte layer 15/the counter electrode layer (cathode) 7) is seenas a single gas decomposition apparatus 10, the gas decompositionapparatus 10 includes an internal resistance R_(in). Accordingly, whenthe gas decomposition apparatus 10 is being operated and current I ispassed through the gas decomposition apparatus 10, a voltage drop ofR_(in)×I occurs at the internal resistance R_(in). In general, sincethere are a plurality of points contributing to the internal resistance,a voltage drop occurs at each point and the total of the voltage dropsis R_(in)×I. As a result, voltage V_(ef) that is effectively applied tothe gas decomposition apparatus 10 is V_(ef)=V−R_(in)×I. The internalresistance R_(in), considerably varies in accordance with the materialof the electrolyte, the thickness of the electrolyte, the state in whichthe catalytic electrode layer (anode) 6, the counter electrode layer(cathode) 7, and the electrolyte layer 15 are in contact with eachother, and the like. Even in electrochemical systems of a single type,the internal resistance R_(in) can vary in accordance with productionchance and from lot to lot.

By measuring potentials at points in the (catalytic electrode layer(anode) 6/electrolyte layer 15/the counter electrode layer (cathode) 7),that is, by determining the distribution of potentials, the degree ofcontribution of each component to the internal resistance can bedetermined. In addition, the voltage V_(ef) that effectively contributesto the electrochemical reaction of the gas decomposition apparatus 10can also be determined. In the measurement of potential, a potentiostatincluding a reference electrode of platinum (Pt), silver (Ag), or thelike is used. Furthermore, it is necessary to adjust influencing factorssuch as temperature to standard conditions. Only in this case,meaningful results that are comparable with other measurement data(potential values) were obtained. Accordingly, “the voltage applied tothe gas decomposition apparatus 10” and the like should not be comparedwith other similar data without thought. In contrast, the source voltageV is at least practically clear. In addition, a voltage source having apredetermined performance (nominal voltage) is provided assuming thatthe voltage source enables actual operation of the gas decompositionapparatus 10 as long as the internal resistance does not excessivelychange. Accordingly, the output voltage or the nominal voltage of avoltage source is an indicator that is practically less likely to causeproblems in comparison with others.

Embodiments of the present invention have been described. However, theembodiments disclosed above are mere examples of the present inventionand the scope of the present invention is not restricted to theseembodiments. The scope of the present invention is defined by thedescriptions of Claims and the present invention encompasses all themodifications falling within the meaning and scope equivalent to thedescriptions of Claims.

INDUSTRIAL APPLICABILITY

According to a gas decomposition apparatus and a gas decompositionmethod according to the present invention, gases having highdecomposition voltages such as aromatic compounds can be rapidlydecomposed without the possibility of the generation of carbon monoxideand gases having low decomposition voltages can be decomposed at a highdecomposition rate. Therefore, the present invention is expected tocontribute to rapid decomposition of odors in places crowded withpeople, indoors, and the like.

REFERENCE SIGNS LIST

-   3 stretched porous PTFE membrane-   5 PFC polymer-   6 catalytic electrode layer (decomposition-side electrode layer)-   6 d conductive base of catalytic electrode layer-   6 g conductive base of carbon fiber-   6 m porous metal sheet (conductive base of metal fiber or the like)-   7 counter electrode layer (catalytic electrode layer)-   8, 9 gas-diffusion layer-   10 gas decomposition apparatus-   11 fine catalytic particles-   15 electrolyte layer-   16 porous sheet-   21 carrier (powder)-   21 m metal powder-   21 d conductive carbon powder-   29 pore-   30 conductive diamond-   33 powder inert to water (Ti₄O₇ powder or PbO₂ powder)-   45 ionic liquid

1. A gas decomposition apparatus comprising: a catalytic electrode thatcontains a catalyst and is porous; a counter electrode that forms a pairwith the catalytic electrode; and an electrolyte that is sandwichedbetween the catalytic electrode and the counter electrode and has ionconductivity, wherein the catalyst is held by the catalytic electrode ina form of being carried by a carrier containing a conductive material orthe catalyst is directly carried by the catalytic electrode, and theconductive material in the catalytic electrode, the conductive materialbeing in contact with the catalyst, is not a noncovalent carbonmaterial.
 2. The gas decomposition apparatus according to claim 1,wherein the conductive material in the catalytic electrode is a porousmetal and at least a portion of the porous metal is formed by a platingprocess.
 3. The gas decomposition apparatus according to claim 2,wherein the catalyst is carried by the porous metal through a resinhaving proton permeability.
 4. The gas decomposition apparatus accordingto claim 2, wherein the porous metal has a porosity of 0.6 or more and0.98 or less.
 5. The gas decomposition apparatus according to claim 2,wherein the porous metal is formed by plating a resin formed by afoaming treatment for forming a large number of bubbles in the resin anda joining pore-formation treatment for joining the bubbles to formpores.
 6. The gas decomposition apparatus according to claim 1, whereina voltage source with which a voltage of 0.8 V or higher can be appliedis provided.
 7. The gas decomposition apparatus according to claim 1,wherein a voltage source with which a voltage of 0.8 V or higher can beapplied between the catalytic electrode and the counter electrode isprovided; and, in a state in which a voltage of 0.8 V or higher isapplied between the catalytic electrode and the counter electrode byusing the voltage source, carbon monoxide is not generated.
 8. The gasdecomposition apparatus according to claim 1, wherein a voltage sourcewith which a voltage of 1.5 V or higher can be applied is provided. 9.The gas decomposition apparatus according to claim 1, wherein a voltagesource with which a voltage of 1.5 V or higher can be applied betweenthe catalytic electrode and the counter electrode is provided; and, in astate in which a voltage of 1.5 V or higher is applied between thecatalytic electrode and the counter electrode by using the voltagesource, carbon monoxide is not generated.
 10. The gas decompositionapparatus according to claim 1, wherein the conductive material being incontact with the catalyst is limited to metal and/or conductive diamond.11. The gas decomposition apparatus according to claim 1, wherein thecarrier is a conductive-diamond-coated powder that is a powder coatedwith conductive diamond; or a metal powder.
 12. The gas decompositionapparatus according to claim 1, wherein the catalytic electrode includesa conductive-diamond-coated porous sheet that is a porous materialcoated with conductive diamond; or a porous metal sheet.
 13. The gasdecomposition apparatus according to claim 1, wherein the catalyst is aplatinum group catalyst.
 14. The gas decomposition apparatus accordingto claim 1, wherein the electrolyte mainly includes a uniaxially orbiaxially stretched porous polytetrafluoroethylene (PTFE) membrane and aperfluorocarbon ion exchange polymer that fills gaps of the porous PTFEmembrane and extends from the catalytic electrode to the counterelectrode.
 15. The gas decomposition apparatus according to claim 1,wherein the electrolyte has a thickness of 50 μm or less.
 16. The gasdecomposition apparatus according to claim 14, wherein a surface of afiber of the porous PTFE membrane is covered with a hydrophilic resinfilm.
 17. A method for decomposing a gas, comprising applying a voltagebetween a catalytic electrode containing a catalyst and a counterelectrode, wherein a membrane electrode assembly (MEA) structure inwhich a conductive material that is in contact with the catalyst in thecatalytic electrode does not include noncovalent carbon materials isprepared; and the gas is decomposed by applying the voltage between thecatalytic electrode and the counter electrode without generation ofcarbon monoxide.
 18. The method for decomposing a gas according to claim17, wherein the gas to be decomposed includes an aromatic compound gas.19. A gas decomposition apparatus comprising: a catalytic electrode inwhich oxidation occurs, the catalytic electrode containing finecatalytic particles; a counter electrode that forms a pair with thecatalytic electrode; and an electrolyte sandwiched between the catalyticelectrode and the counter electrode, wherein the electrolyte is anonaqueous electrolyte.
 20. The gas decomposition apparatus according toclaim 19, wherein the electrolyte includes (1) an ionic liquidfunctioning at room temperature or (2) CsHSO₄, a molten salt, or a solidoxide electrolyte functioning under heating.
 21. The gas decompositionapparatus according to claim 19, wherein the electrolyte is a solidmembrane containing an ionic liquid.
 22. The gas decomposition apparatusaccording to claim 19, wherein a voltage source with which a voltage of1.5 V or higher can be applied is provided.
 23. The gas decompositionapparatus according to claim 19, wherein a voltage source with which avoltage of 1.5 V or higher can be applied between the catalyticelectrode and the counter electrode is provided.
 24. The gasdecomposition apparatus according to claim 19, wherein a conductivematerial that is one of main constituent materials of the catalyticelectrode and is in contact with the fine catalytic particles so thatelectrons generated by a gas decomposition reaction are conductedthrough the conductive material is not a noncovalent carbon material.25. The gas decomposition apparatus according to claim 19, wherein aconductive material being in contact with the fine catalytic particlesin the catalytic electrode is limited to metal and/or conductivediamond.
 26. A gas decomposition apparatus comprising: adecomposition-side electrode being a porous electrode into which gascontaining a component to be decomposed is introduced; a counterelectrode that forms a pair with the decomposition-side electrode and isporous; and an electrolyte sandwiched between the decomposition-sideelectrode and the counter electrode, wherein a portion of thedecomposition-side electrode, the portion being in contact with theelectrolyte, is composed of a material that is inert to water.
 27. Thegas decomposition apparatus according to claim 26, wherein a voltagesource with which a voltage of 1.23 V or higher can be applied isprovided.
 28. The gas decomposition apparatus according to claim 26,wherein a voltage source with which a voltage of 1.23 V or higher can beapplied is provided; the electrolyte is an electrolyte containing water;and, in a state in which a voltage of 1.23 V or higher is appliedbetween the decomposition-side electrode and the counter electrode byusing the voltage source, the water in the electrolyte is notdecomposed.
 29. The gas decomposition apparatus according to claim 26,wherein the decomposition-side electrode is covered with conductivediamond or contains any one of a conductive oxide, a conductive nitride,and a conductive sulfide.
 30. The gas decomposition apparatus accordingto claim 29, wherein the decomposition-side electrode contains Ti₄O₇ orPbO₂.
 31. The gas decomposition apparatus according to claim 26, whereinthe decomposition-side electrode includes a porous sheet covered withconductive diamond on which Ti₄O₇ powder and/or PbO₂ powder is held. 32.A method for decomposing a gas, comprising applying a voltage between adecomposition-side electrode and a counter electrode that sandwich anelectrolyte therebetween, and introducing gas containing a component tobe decomposed into the decomposition-side electrode to decompose thecomponent, wherein the decomposition-side electrode contains a materialthat is inert to water and the electrolyte contains an electrolytecontaining water; and a voltage of 1.23 V or higher is applied betweenthe decomposition-side electrode and the counter electrode so that thecomponent is decomposed without decomposing the water in theelectrolyte.